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CBO Costing Plans With Migrated Rows Part I (“Ignoreland”) March 21, 2023

Posted by Richard Foote in BLEVEL, CBO, Clustering Factor, Data Clustering, Index Access Path, Index Height, Index statistics, Leaf Blocks, Migrated Rows, Non-Equality Predicates, Oracle, Oracle Blog, Oracle Cost Based Optimizer, Oracle General, Oracle Indexes, Oracle Statistics, Performance Tuning, Richard's Blog, ROWID.
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Whilst recently blogging about Migrated Rows and specifically changes to how ROWIDs are now maintained on the fly in Oracle Autonomous Databases, I made a discovery regarding how the Cost-Based Optimizer (CBO) costs such plans. This is one of the key reasons why I blog, not only to try and share odd titbits about how Oracle works, but also to hopefully learn much myself in the process.

Imagine my surprise in not only learning that Oracle and the CBO works differently to how I had always thought Oracle worked in this respect, but that this behaviour has been the case since at least Oracle 9i.

In Part I, I’ll use the same example of migrated rows as I’ve used in the past few blog posts and initially show how the CBO generally costs such plans (and by which I had incorrectly assumed ALWAYS costed such plans).

Let’s start by creating and populating a tightly packed table (in an environment where ROWIDs are NOT updated on the fly):

SQL> create table bowie(id number, code1 number, code2 number, code3 number, code4 number, code5 number, code6 number, code7 number, code8 number, code9 number, code10 number, code11 number, code12 number, code13 number, code14 number, code15 number, code16 number, code17 number, code18 number, code19 number, code20 number, name varchar2(142)) PCTFREE 0;

Table BOWIE created.

SQL> insert into bowie SELECT rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, 'BOWIE' FROM dual CONNECT BY LEVEL <= 200000;

200,000 rows inserted.

SQL> commit;

Commit complete.

I’ll next create an index on the well clustered ID column (as the rows are inserted in ID column order within the table):

SQL> create index bowie_id_i on bowie(id);

Index BOWIE_ID_I created.

Next, we’ll use the Oracle recommended method of collecting table/index statistics, by using the DBMS_STATS package:

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

SQL> select table_name, num_rows, blocks, empty_blocks, avg_space, avg_row_len, chain_cnt from user_tables
where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS    EMPTY_BLOCKS    AVG_SPACE    AVG_ROW_LEN    CHAIN_CNT
_____________ ___________ _________ _______________ ____________ ______________ ____________
BOWIE              200000      3268               0            0            111            0

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE';

   INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
_____________ _________ ______________ ____________________
BOWIE_ID_I            1            473                 3250

 

Note the key index statistics here: BLEVEL=1, LEAF_BLOCKS=473 and the near perfect CLUSTERING_FACTOR=3250.

If we run the following query featuring a non-equality range predicate:

 

SQL> select * from bowie where id > 1 and id < 1001;

999 rows selected.

PLAN_TABLE_OUTPUT
_______________________________________________________________________________________________________________
SQL_ID b1vwpu2rgn8p5, child number 0
-------------------------------------
select * from bowie where id > 1 and id < 1001

Plan hash value: 1405654398

------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows | A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |    999 |00:00:00.01 |      18 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   1000 |    999 |00:00:00.01 |      18 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   1000 |    999 |00:00:00.01 |       4 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">1 AND "ID"<1001)

Statistics
-----------------------------------------------------------
          1 CPU used by this session
          1 CPU used when call started
          1 DB time
       7678 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
         16 buffer is not pinned count
       1983 buffer is pinned count
        323 bytes received via SQL*Net from client
     171383 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
         18 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
         18 consistent gets from cache
         17 consistent gets pin
         17 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
     147456 logical read bytes from cache
         17 no work - consistent read gets
         40 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          2 process last non-idle time
          1 session cursor cache count
          1 session cursor cache hits
         18 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
        999 table fetch by rowid
          3 user calls

We notice that the CBO indeed uses the index.

They key statistic to note here is that Consistent Gets is just 18, which is extremely low considering we’re returning 999 rows. This is due to the fact the index is currently extremely efficient as it can fetch multiple rows by visiting the same table block due to the excellent clustering/ordering of the required ID column values (and also due to my high arraysize session setting).

If we look at the CBO costings for this plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'b1vwpu2rgn8p5',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
____________________________________________________________________________________________________________________________________
SQL_ID b1vwpu2rgn8p5, child number 0

-------------------------------------

select * from bowie where id > 1 and id < 1001

Plan hash value: 1405654398

---------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     |Buffers |
---------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |       |    21 (100)|    999 |00:00:00.01 |     18 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   1000 |   108K|      21 (0)|    999 |00:00:00.01 |     18 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   1000 |       |       4 (0)|    999 |00:00:00.01 |      4 |
---------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">1 AND "ID"<1001)

 

I’ve previously discussed many times how the CBO costs index access paths, but it’s always useful to go over this again, as it’s the most common question I get asked when I visit customer sites.

The KEY statistic the CBO has to determine is the estimated Selectivity of the query (the estimated percentage of rows to be returned), as this is the driver of all the subsequent CBO calculations.

The Selectivity of this range-based predicate query is calculated as follows:

Selectivity = (Highest Bound Value – Lowest Bound Value) / (Highest Value – Lowest Value)
= (1001-1) /(200000-1)
= 1000/199999
=  approx. 0.005

Once Oracle has the selectivity, it can calculate the query Cardinality (estimated number of rows) as follows:

Cardinality = Selectivity x No of Rows

Cardinality = 0.005 x 200000 = 1000 rows

This is our visual window into the likelihood that the CBO has made an accurate decision with its execution plan. If the cardinality estimates are reasonably accurate, then the CBO is likely to generate a good plan. If the cardinality estimates are way off, then the CBO is more likely to generate an inappropriate plan.

The CBO cardinality estimate in the above plan is 1000 rows, whereas the number of rows actually returned is 999 rows.

So indeed, the CBO has got the cardinality almost spot on (except for a trivial rounding error) and so we have a high degree of confidence that the CBO is using the correct selectivity estimates when they get plugged into the following CBO formula for costing an index range scan (using this selectivity of 0.005 and the index statistics listed above):

Index Scan Cost = (blevel + ceil(effective index selectivity x leaf_blocks)) + ceil(effective table selectivity x clustering_factor)

= (1 + ceil(0.005 x 467)) + ceil(0.005 x 3250)
= (1 + 3) + 17
= 4 + 17 = 21

So we can clearly see where the CBO gets its costings for both reading the index during the Index Range Scan (4) and for the plan as a whole (21).

The CBO cost of 21 very closely resembles the 18 consistent gets accessed when the plan is executed. This to me suggests that the CBO has indeed costed this plan very accurately and appropriately.

It’s interesting to note in the above execution plan that Oracle is attributing 100% of this cost of 21 to CPU (21 (100)). That will be a discussion for another day…

OK, let’s now perform an update on the table, increasing the size of the rows such that I generate a bunch of migrated rows:

SQL> update bowie set name='THE RISE AND FALL OF BOWIE STARDUST AND THE SPIDERS FROM MARS';

200,000 rows updated.

SQL> commit;

Commit complete.

If we now collect fresh statistics again using DBMS_STATS:

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

SQL> select table_name, num_rows, blocks, empty_blocks, avg_space, avg_row_len, chain_cnt from user_tables
where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS    EMPTY_BLOCKS    AVG_SPACE    AVG_ROW_LEN    CHAIN_CNT
_____________ ___________ _________ _______________ ____________ ______________ ____________
BOWIE              200000      4906               0            0            167            0

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE';

   INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
_____________ _________ ______________ ____________________
BOWIE_ID_I            1            473                 3250

 

We notice that none of the key statistics have changed, except for the number of Table Blocks (now 4906, previously it was 3268) and the Average Row Length has also increased (now 167, previously it was 111). Both of these can of course be attributed to the increase in the size of the values now stored in the NAME column following the Update.

Importantly, notice that collecting statistics via DBMS_STATS does NOT collect data for the CHAIN_CNT statistic, it remains at 0 even though many migrated rows were actually generated by the Update statement (as we’ll see below).

Increasing the Table Blocks will result in an associated increase in the cost of reading this table via a Full Table Scan (FTS).

We notice that none of the index-related statistics changed following the Update statement (as in this example, Oracle does NOT update the ROWIDs of any of the migrated rows, Oracle simply stores a pointer in the original block to denote the new physical location of the migrated rows as previously discussed).

So if we only INCREASE the cost of a FTS (via having more Table Blocks) but keep intact all the previous index related statistics, then the CBO is certainly going to again select the same Index Range Scan plan, as the plan will have the same (cheaper than FTS) costings as before.

If we re-run the query again:

SQL> select * from bowie where id > 1 and id < 1001;

999 rows selected.

PLAN_TABLE_OUTPUT
_______________________________________________________________________________________________________________
SQL_ID b1vwpu2rgn8p5, child number 0
-------------------------------------
select * from bowie where id > 1 and id < 1001

Plan hash value: 1405654398

------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows | A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |    999 |00:00:00.01 |     666 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   1000 |    999 |00:00:00.01 |     666 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   1000 |    999 |00:00:00.01 |       4 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">1 AND "ID"<1001)

Statistics
-----------------------------------------------------------
          1 CPU used by this session
          1 CPU used when call started
       7709 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
        664 buffer is not pinned count
       1662 buffer is pinned count
        323 bytes received via SQL*Net from client
     171500 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
        666 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
        666 consistent gets from cache
        665 consistent gets pin
        665 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
    5455872 logical read bytes from cache
        665 no work - consistent read gets
         39 non-idle wait count
          1 non-idle wait time
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          2 session cursor cache count
        666 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
        999 table fetch by rowid
        327 table fetch continued row
          3 user calls

We notice that indeed it’s the same Index Range Scan plan as before.

But we notice that the number of Consistent Gets has increased substantially to 666 (previously it was just 18). The reason for this large jump is due to the now 327 table fetch continued rows that need to be accessed due to the newly migrated rows following the Update. This number is then doubled (so 2 x 327 = 654) to represent the approximate additional Consistent Gets we now need to perform, as Oracle needs to read the additional table block to access the migrated row’s new physical location AND to now re-read the original table block to access the next row to be fetched (previously Oracle could read all the required consecutive rows required from the same table block within the one consistent get).

So it’s now actually substantially more expensive to read the required 1000 rows via this index due to this increase in necessary consistent gets.

But if we look at the actual cost of this plan now:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'b1vwpu2rgn8p5',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
____________________________________________________________________________________________________________________________________
SQL_ID b1vwpu2rgn8p5, child number 0

-------------------------------------

select * from bowie where id > 1 and id < 1001

Plan hash value: 1405654398

---------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     |Buffers |
---------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |       |    21 (100)|    999 |00:00:00.01 |    666 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   1000 |   163K|      21 (0)|    999 |00:00:00.01 |    666 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   1000 |       |       4 (0)|    999 |00:00:00.01 |      4 |
---------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">1 AND "ID"<1001)

 

We notice that as expected (as none of the index-related statistics have changed), that despite being much more expensive to now use this index, the costs of this plan (4 for reading the index and 21 overall) remain unchanged.

I would argue that these CBO costs are no longer as accurate as the 21 total CBO cost does not so closely represent the actual 666 consistent gets now required.

Now, the 327 table fetch continued row statistics from the previous run is clear proof we indeed have migrated rows following the Update statement.

But if we want to confirm how many migrated rows we now have in the table, we can use the ANALYZE command to collect these additional statistics:

SQL> analyze table bowie compute statistics;

Table BOWIE analyzed.

SQL> select table_name, num_rows, blocks, empty_blocks, avg_space, avg_row_len, chain_cnt from user_tables
where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS    EMPTY_BLOCKS    AVG_SPACE    AVG_ROW_LEN    CHAIN_CNT
_____________ ___________ _________ _______________ ____________ ______________ ____________
BOWIE              200000      4906              86          415            170        56186

 

We notice that we now have a CHAIN_CNT of 56186.

Now this statistic can represent any row that is not housed inside a single table block (for which there could be a number of possible reasons, such as a row simply being too long to fit in a single table block), but as all rows are still relatively tiny, we can be certain that indeed all 56186 chained rows represent migrated rows.

Now that I’ve gone and used ANALYZE, primarily to generate this CHAIN_CNT statistic, my previous understanding of how the CBO costs migrated rows crumbles away, as I’ll discuss in my next post…

Migrated Rows In Oracle Data Warehouse Autonomous Databases (“Sit Down. Stand Up.”) March 14, 2023

Posted by Richard Foote in 19c, 19c New Features, Autonomous Data Warehouse, Autonomous Database, Changing ROWID, Migrated Rows, Oracle, Oracle Cloud, Oracle Indexes, Oracle19c, ROWID.
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In all my recent discussions on how Oracle can now update ROWIDs on the fly when a row migrates, I’ve mentioned how this only occurs on tables in which the ENABLE ROW MOVEMENT clause has been set.

So you have the option on whether you wish this new behaviour to occur by simply not setting ENABLE ROW MOVEMENT on tables where you want the previous behavior of the ROWIDs not changing when a row migrates (and for Oracle to simply have a pointer in the original table block to denote the new location of the row). You may not what ROWIDs to suddenly change on you for example if you have an application that explicitly stores ROWIDs and relies on them not changing for the application to correctly fetch data.

However, all my previous tests and examples have been run on Oracle Transaction Processing Autonomous Database environments, but as Phil Goldenberg mentioned in this comment, things unfortunately behave somewhat differently in Oracle Data Warehouse Autonomous Database environments.

To illustrate, a simple little demo as usual, but this time using an Oracle Data Warehouse Autonomous Database environment…

Let’s start by creating and populating a tightly packed table, but without setting the ENABLE ROW MOVEMENT clause:

SQL> create table bowie (id number, name varchar2(142)) pctfree 0;

Table BOWIE created.

SQL> insert into bowie select rownum, 'BOWIE' from dual connect by level <=10000;

10,000 rows inserted.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE');

PL/SQL procedure successfully completed.

Let’s now create an index based on the ID column:

SQL> create index bowie_id_i on bowie(id);

Index BOWIE_ID_I created.

SQL> select index_name, num_rows, blevel, leaf_blocks from user_indexes where table_name='BOWIE';

   INDEX_NAME    NUM_ROWS    BLEVEL    LEAF_BLOCKS
_____________ ___________ _________ ______________
BOWIE_ID_I          10000         1             23

Let’s have a look at the ROWIDs of a few random rows:

SQL> select id, rowid from bowie where id in (42, 424, 4242) order by id;

     ID                 ROWID
_______ _____________________
     42 AAASTdAAAAAAJq0AAp
    424 AAASTdAAAAAAJq0AIH
   4242 AAASTdAAAAAAJq1ADP

If we store these ROWIDs, we can use them to directly access a row of interest very efficiently:

SQL> select id from bowie where rowid='AAASTdAAAAAAJq0AAp';

   ID
_____
   42

Next, I’ll update the table, increasing the size of the rows such that I generate a bunch of migrated rows:

SQL> update bowie set name='THE RISE AND FALL OF ZIGGY STARDUST AND THE SPIDERS FROM MARS';

10,000 rows updated.

SQL> commit;

Commit complete.

As discussed previously, if this were an Oracle Transaction Processing Autonomous Database environment, because I haven’t set ENABLE ROW MOVEMENT on this table, the ROWIDs of any migrated rows would NOT have changed.

But here in this Oracle Data Warehouse Database environment:

SQL> select id, rowid from bowie where id in (42, 424, 4242) order by id;

     ID                 ROWID
_______ _____________________
     42 AAASTdAAAAAAJq3AAp
    424 AAASTdAAAAAAJq3AIH
   4242 AAASTdAAAAAAJrcAFd

We can see that all these rows now have a new ROWID.

If I now re-run my previous query that relied on the ROWIDs not changing:

SQL> select id from bowie where rowid='AAASTdAAAAAAJq0AAp';

no rows selected

We can see that the query no longer returns the required row.

And yet, if we ANALYZE the table:

SQL> analyze table bowie compute statistics;

Table BOWIE analyzed.

SQL> select table_name, num_rows, blocks, chain_cnt, row_movement from user_tables where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS    CHAIN_CNT    ROW_MOVEMENT
_____________ ___________ _________ ____________ _______________
BOWIE               10000        28            0 DISABLED

We can see that we have no migrated/chained rows listed, even though ENABLE ROW MOVEMENT is disabled.

I think this a little unfortunate, because I quite like the concept of ROWIDs being updated when a row migrates, but I also like the option of being able to revert to the previous behaviour if necessary.

I have no idea how this works in the other Oracle Autonomous Database environments (other than Transaction Processing), but regardless, this behaviour can potentially change on any of the environments at any time.

So, what’s the moral of the story? Well there’s a couple.

Firstly, in Oracle Autonomous Database environments, you’re meant to have a “hands-off” attitude and just let Oracle handle all this day to day stuff. So you can’t necessarily rely on the behaviour of the database to be as consistent as in environments where you control all the levers. Indexes might come and go, tables might suddenly get partitioned, ROWIDs might change when a row migrates, etc. etc. etc. etc.

And secondly, it’s becoming an even worse idea for applications to explicitly store and rely on ROWIDs not changing for such applications function properly. Especially, if you use Oracle Autonomous Database environments…

Annoying Bug When Trying To Create Always Free TP Autonomous Database (“Supernaturally”) March 10, 2023

Posted by Richard Foote in Autonomous Database, Autonomous Transaction Processing, Create Autonomous Database Bug, Oracle, Oracle Blog, Oracle Bugs.
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I was having all sorts of problems with one of my Always Free Oracle Transaction Processing Autonomous Databases, mostly around space and configuration issues within its parent Container Database, that I just gave up, blew it away and just tried to create a fresh one. That’s the beauty of this great free capability, it’s so easy to just throw away and create new databases in only a few moments.

Well, usually it’s easy…

I now encounter an issue trying to create a new Always Free autonomous database via a really annoying bug. I thought I’ll share the issue (and the fix) in case anyone else encounters this issue (which was in the Australia East Sydney data centre).

So I go into OCI, then into the Autonomous Database screen and select Create Autonomous Database which gets you into the following screen (if you don’t know how to get into this screen, this post is likely to be of no interest to you):

 

So you fill in the new Database details and then scroll down:

 

 

Now if you select you want to create a Transaction Processing database and scroll down:

 

We now hit the damn bug. For some reason, when you select the Transaction Processing option, the OCPU auto scaling option gets selected and there’s no way to turn the thing off (you can’t just click on the greyed tick icon).

Which is unfortunate because this option is NOT allowed if you want to create an ALWAYS FREE Autonomous Database.

If you scroll down and continue to create your always free autonomous database by clicking on the Create Autonomous Database button:

You get an error saying you can’t because you’re using an option that’s not permitted with always free databases. The OCPU auto scaling option that you can’t now turn off!!!

However, there is a way to get out of this pickle and create the always free autonomous database. Firstly you need to scroll up again:

 

 

And unselect the Show only Always Free configuration options button.

This now makes the OCPU auto scaling tick button available to be unselected.

 

So now you can turn off the damn OCPU auto scaling option that you never wanted in the first place.

 

 

Now don’t forget to go back and select the Show only Always Free configuration options button again to ensure you don’t in fact use any options or sizings that prevents the creation of an Always Free autonomous database.

Alternatively, you can just toggle the Show only Always Free configuration options button, as turning it back on again will also remove the OCPU auto scaling tick. But if you do this, make sure you’ve selected which type of autonomous database you want, as changing the selection will automatically re-enable OCPU auto scaling and you’ll have to repeat the fixing process.

 

 

You can now finally click the Create Autonomous Database at the bottom of the screen and successfully create your free always free autonomous database.

So it’s a pain, but at least I found a relatively simple workaround.

As I mentioned previously, I encounter this issue when connected to the Australia East Sydney data centre. I have no idea if this occurs in other Oracle data centres as well?

I don’t currently have a Support Identifier, so if you do or you work for Oracle Corporation, feel free to raise a Service Request for this issue… 🙂

 

UPDATE 15 March 2023: This issue has now been resolved by Oracle. A huge thank you to Yasin Baskan for so very promptly addressing this issue.

Possible Impact To Clustering Factor Now ROWIDs Are Updated When Rows Migrate Part III (“Dancing With The Big Boys”) March 9, 2023

Posted by Richard Foote in 19c, 19c New Features, Attribute Clustering, Autonomous Data Warehouse, Autonomous Database, Autonomous Transaction Processing, CBO, Changing ROWID, Clustering Factor, Data Clustering, Full Table Scans, Index Access Path, Index Internals, Index Rebuild, Index statistics, Leaf Blocks, Migrated Rows, Oracle, Oracle 21c, Oracle Blog, Oracle Cloud, Oracle Cost Based Optimizer, Oracle General, Oracle Indexes, Oracle19c, ROWID.
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In my previous post, I discussed how you can best reorg a table that has a significant number of migrated rows impact the Clustering Factor of important indexes, when such tables have the ENABLED ROW MOVEMENT disabled.

In this post I’ll discuss resolving similar issues, but when ROWIDs are updated on the fly when rows are migrated in Oracle Autonomous Databases.

As I discussed previously, by updating indexes with the new ROWIDs when rows migrate, such indexes can potentially increase in size as they store both old/new index entries concurrently AND due to the increased likelihood of associated index block splits. Additionally, such indexes can also have their Clustering Factor directly impacted when migrated rows disrupt the otherwise tight clustering of specific columns.

As such, we may want to address these issues to improve the performance of impacted queries.  But it’s important we address these issues appropriately…

To illustrate all this, I’m going to re-run the same demo as my previous post, but on a table with ENABLE ROW MOVEMENT enabled.

I’ll start by creating and populating a tightly packed table with ENABLE ROW MOVEMENT enabled and with data inserted in ID column order:

SQL> create table bowie2(id number, code1 number, code2 number, code3 number, code4 number, code5 number, code6 number, code7 number, code8 number, code9 number, code10 number, code11 number, code12 number, code13 number, code14 number, code15 number, code16 number, code17 number, code18 number, code19 number, code20 number, name varchar2(142)) PCTFREE 0 ENABLE ROW MOVEMENT;

Table BOWIE2 created.

SQL> insert into bowie2 SELECT rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, 'BOWIE' FROM dual CONNECT BY LEVEL <= 200000;

200,000 rows inserted.

SQL> commit;

Commit complete.

I’ll now create an index on this well ordered/clustered ID column:

SQL> create index bowie2_id_i on bowie2(id);

Index BOWIE2_ID_I created.

Next, I’ll update the table, increasing the size of the rows such that I generate a bunch of migrated rows:

SQL> update bowie2 set name='THE RISE AND FALL OF BOWIE STARDUST AND THE SPIDERS FROM MARS';

200,000 rows updated.

SQL> commit;

Commit complete.

 

If we check the number of migrated rows:

SQL> analyze table bowie2 compute statistics;

Table BOWIE2 analyzed.

SQL> select table_name, num_rows, blocks, empty_blocks, avg_space, avg_row_len, chain_cnt from user_tables where table_name='BOWIE2';

   TABLE_NAME    NUM_ROWS    BLOCKS    EMPTY_BLOCKS    AVG_SPACE    AVG_ROW_LEN    CHAIN_CNT
_____________ ___________ _________ _______________ ____________ ______________ ____________
BOWIE2             200000      4654              82          367            169            0

We notice there are indeed 0 migrated rows. This is because in Oracle Autonomous Databases, the associated ROWIDs of migrated rows as updated on the fly in this scenario.

If we check the current Clustering Factor of the index:

SQL> execute dbms_stats.delete_table_stats(ownname=>null, tabname=>'BOWIE2');

PL/SQL procedure successfully completed.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE2', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

SQL> select table_name, num_rows, blocks from user_tables where table_name='BOWIE2';

   TABLE_NAME    NUM_ROWS    BLOCKS
_____________ ___________ _________
BOWIE2             200000      4654

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE2';

    INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
______________ _________ ______________ ____________________
BOWIE2_ID_I            2            945               109061

We can see that although the data was initially inserted in ID column order, we now have a relatively poor Clustering Factor at 109061 as the migrated rows have disrupted this previously perfect clustering.

We also notice that the BLEVEL has increased from 1 to now be 2 and the number of Leaf Blocks has increased to 945 from 473 after the rows migrated (as I discussed previously).

If we now run a query that returns 4200 rows from a 200,000 row table:

SQL> select * from bowie2 where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 4200

Plan hash value: 1495904576

----------------------------------------------------------------------------------------------
| Id | Operation                  | Name   | Starts | E-Rows | A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT           |        |      1 |        |   4200 |00:00:00.02 |    4572 |
|* 1 |  TABLE ACCESS STORAGE FULL | BOWIE2 |      1 |   4200 |   4200 |00:00:00.02 |    4572 |
----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - storage(("ID"<=4200 AND "ID">=1))
       filter(("ID"<=4200 AND "ID">=1))

Note
-----
    - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          4 CPU used by this session
          4 CPU used when call started
          4 DB time
      37101 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
          2 buffer is not pinned count
        325 bytes received via SQL*Net from client
     461965 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
         14 calls to kcmgcs
       4572 consistent gets
       4572 consistent gets from cache
       4572 consistent gets pin
       4572 consistent gets pin (fastpath)
          2 execute count
   37453824 logical read bytes from cache
       4560 no work - consistent read gets
         72 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          1 session cursor cache count
          1 session cursor cache hits
       4572 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4560 table scan blocks gotten
     252948 table scan disk non-IMC rows gotten
     252948 table scan rows gotten
          1 table scans (short tables)
          3 user calls

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'25qktyn35b662',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
______________________________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 4200

Plan hash value: 1495904576

-------------------------------------------------------------------------------------------------------------------
| Id | Operation                  | Name   | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT           |        |      1 |        |       |  1264 (100)|   4200 |00:00:00.02 |    4572 |
|* 1 |  TABLE ACCESS STORAGE FULL | BOWIE2 |      1 |   4200 |   684K|    1264 (1)|   4200 |00:00:00.02 |    4572 |
-------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - storage(("ID"<=4200 AND "ID">=1))
       filter(("ID"<=4200 AND "ID">=1))

 

We can see that Oracle has decided to perform a Full Table Scan (FTS) and not use the index.

The Clustering Factor of the ID column is now so bad, that returning 4200 rows via such an index is just too expensive. The FTS is now deemed the cheaper option by the CBO.

We notice that the CBO cost of the FTS is 1264.

If we run a query that forces the use of the index:

SQL> select /*+ index (bowie2) */ * from bowie2 where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
________________________________________________________________________________________________________________
SQL_ID bzm2vhchqpq7w, child number 0
-------------------------------------
select /*+ index (bowie2) */ * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

-------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows | A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |   4200 |00:00:00.01 |    2665 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   4200 |00:00:00.01 |    2665 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      21 |
-------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)


Statistics
-----------------------------------------------------------
          2 CPU used by this session
          2 CPU used when call started
          2 DB time
      14531 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
       2646 buffer is not pinned count
       5755 buffer is pinned count
        348 bytes received via SQL*Net from client
     462143 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
       2665 consistent gets
          2 consistent gets examination
          2 consistent gets examination (fastpath)
       2665 consistent gets from cache
       2663 consistent gets pin
       2663 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
   21831680 logical read bytes from cache
       2663 no work - consistent read gets
         73 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          3 process last non-idle time
          2 session cursor cache count
       2665 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4200 table fetch by rowid
          3 user calls

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'bzm2vhchqpq7w',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________________________________________
SQL_ID bzm2vhchqpq7w, child number 0

-------------------------------------

select /*+ index (bowie2) */ * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |       |  2314 (100)|   4200 |00:00:00.01 |    2665 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   684K|    2314 (1)|   4200 |00:00:00.01 |    2665 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |       |      22 (0)|   4200 |00:00:00.01 |      21 |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

 

The cost of the Index Range Scan plan has an overall cost of 2314, greater than the 1264 cost of the FTS plan.

Notice that the cost of using just the index within the plan is currently 22.

So the vast majority of the cost of this plan (2314 – 22 = 2292) is in Oracle having to access so many different table blocks due to the poor index Clustering Factor and NOT in the increased size of the index.

As I’ve discussed numerous times, you can potentially make an index smaller by rebuilding the index (if there’s free space within the index), but the impact on the Clustering Factor will be nothing but “disappointing”…

If we just rebuild the index:

SQL> alter index bowie2_id_i rebuild online;

Index BOWIE2_ID_I altered.

And now look at the new index related statistics:

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE2';

    INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
______________ _________ ______________ ____________________
BOWIE2_ID_I            1            473               109061

We notice that the index has indeed decreased in size, back to what is was before the row migrated following the Update (Blevel=1 and Leaf Blocks=473).

But the Clustering Factor remains unchanged at 109061.

If we now re-run the query:

 

SQL> select * from bowie2 where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 4200

Plan hash value: 1495904576

----------------------------------------------------------------------------------------------
| Id | Operation                  | Name   | Starts | E-Rows | A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT           |        |      1 |        |   4200 |00:00:00.02 |    4572 |
|* 1 |  TABLE ACCESS STORAGE FULL | BOWIE2 |      1 |   4200 |   4200 |00:00:00.02 |    4572 |
----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - storage(("ID"<=4200 AND "ID">=1))
       filter(("ID"<=4200 AND "ID">=1))

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          3 CPU used by this session
          3 CPU used when call started
          3 DB time
      31738 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
          2 buffer is not pinned count
        325 bytes received via SQL*Net from client
     461972 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
         14 calls to kcmgcs
       4572 consistent gets
       4572 consistent gets from cache
       4572 consistent gets pin
       4572 consistent gets pin (fastpath)
          2 execute count
   37453824 logical read bytes from cache
       4560 no work - consistent read gets
         73 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          3 process last non-idle time
          2 session cursor cache count
       4572 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4560 table scan blocks gotten
     252948 table scan disk non-IMC rows gotten
     252948 table scan rows gotten
          1 table scans (short tables)
          3 user calls

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'25qktyn35b662',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
______________________________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 4200

Plan hash value: 1495904576

-------------------------------------------------------------------------------------------------------------------
| Id | Operation                  | Name   | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT           |        |      1 |        |       |  1264 (100)|   4200 |00:00:00.02 |    4572 |
|* 1 |  TABLE ACCESS STORAGE FULL | BOWIE2 |      1 |   4200 |   684K|    1264 (1)|   4200 |00:00:00.02 |    4572 |
-------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - storage(("ID"<=4200 AND "ID">=1))
       filter(("ID"<=4200 AND "ID">=1))

 

The CBO decides to still use a FTS instead of the index.

If we look at the cost now of using the index for this query:

SQL> select /*+ index (bowie2) */ * from bowie2 where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
________________________________________________________________________________________________________________
SQL_ID bzm2vhchqpq7w, child number 0
-------------------------------------
select /*+ index (bowie2) */ * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

-------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows | A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |   4200 |00:00:00.01 |    2655 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   4200 |00:00:00.01 |    2655 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      11 |
-------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

Note
-----
- automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          2 CPU used by this session
          2 CPU used when call started
          1 DB time
      13484 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
       2646 buffer is not pinned count
       5755 buffer is pinned count
        347 bytes received via SQL*Net from client
     461972 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
       2655 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
       2655 consistent gets from cache
       2654 consistent gets pin
       2654 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
   21749760 logical read bytes from cache
       2654 no work - consistent read gets
         73 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          1 session cursor cache count
          1 session cursor cache hits
       2655 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4200 table fetch by rowid
          3 user calls

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'bzm2vhchqpq7w',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________________________________________
SQL_ID bzm2vhchqpq7w, child number 0

-------------------------------------

select /*+ index (bowie2) */ * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |       |  2303 (100)|   4200 |00:00:00.01 |    2655 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   684K|    2303 (1)|   4200 |00:00:00.01 |    2655 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |       |      11 (0)|   4200 |00:00:00.01 |      11 |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

 

We notice the cost of the index has only moderately gone down to 2303 (previously it was 2314).

This reduction of 11 in the CBO cost is due entirely to the fact the index is now approximately 1/2 the size as it was before the index rebuild and has thus reduced the cost of reading the index blocks to 11 within the execution plan (previously it was 22).

But the vast majority of the cost within the Index Range Scan plan comes again with accessing the table blocks, which remains unchanged due to the unchanged Clustering Factor.

To reduce the Clustering Factor, we need to change the clustering of the data with the TABLE.

So, to improve the performance of this potentially important query, we need to re-cluster the data just as we did in the example in my previous post when we had migrated rows listed and ROWIDs were not updated on the fly.

We can now add an appropriate Clustering Attribute before we perform the table reorg:

SQL> alter table bowie2 add clustering by linear order (id);

Table BOWIE2 altered.

SQL> alter table bowie2 move online;

Table BOWIE2 altered.

If we now look at the Clustering Factor of this important index:

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE2', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

SQL> select table_name, num_rows, blocks from user_tables where table_name='BOWIE2';

   TABLE_NAME    NUM_ROWS    BLOCKS
_____________ ___________ _________
BOWIE2             200000      4936

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE2';

    INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
______________ _________ ______________ ____________________
BOWIE2_ID_I            1            473                 4850

The Clustering Factor has been reduced down to the almost perfect 4850, down from the previous 109061.

If we now re-run the query:

SQL> select * from bowie2 where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
________________________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

-------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows | A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |   4200 |00:00:00.01 |     102 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   4200 |00:00:00.01 |     102 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      11 |
-------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

Statistics
-----------------------------------------------------------
          1 CPU used by this session
          1 CPU used when call started
         90 Cached Commit SCN referenced
      11345 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
         93 buffer is not pinned count
       8308 buffer is pinned count
        325 bytes received via SQL*Net from client
     462117 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
        102 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
        102 consistent gets from cache
        101 consistent gets pin
        101 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
     835584 logical read bytes from cache
        101 no work - consistent read gets
         72 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          2 process last non-idle time
          1 session cursor cache count
          1 session cursor cache hits
        102 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4200 table fetch by rowid
          3 user calls

 

We can see the query now automatically uses the index and only requires just 102 consistent gets, down from 4572 when it performed the FTS.

If we look at the cost of this new plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'25qktyn35b662',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0

-------------------------------------

select * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |       |   113 (100)|   4200 |00:00:00.01 |     102 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   684K|     113 (0)|   4200 |00:00:00.01 |     102 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |       |      11 (0)|   4200 |00:00:00.01 |      11 |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

 

We can see the plan has a cost of just 113, which is both much more accurate and close to the 102 consistent gets and much less than the previous cost of 1340 for the FTS plan.

So in specific examples where migrated rows significantly impact the Clustering Factor of indexes important to our applications, including when ROWIDs are updated on the fly in Oracle Autonomous Databases, we may need to appropriately reorg such tables to repair the Clustering Factor of impacted indexes.

I’ve mentioned a number of times in this series how tables in Oracle Autonomous Databases with ENABLE ROW MOVEMENT have their ROWIDs updated on the fly when a row migrates. In my next post, I’ll discuss how even tables that don’t have the ENABLE ROW MOVEMENT clause set can still have their ROWIDs updated on the fly when a row migrates…

Oracle ACE Director Award 2022 March 8, 2023

Posted by Richard Foote in Oracle ACE Director, Richard's Musings.
add a comment

 

I received a very nice surprise in the mail today, my Oracle ACE Director Award for 2022.

A big thank you to Jen and the whole team at Oracle who support all the Oracle ACE folks so well.

Hopefully, I’ll have the opportunity to present at an Oracle conference or two sometime in the near future… 🙂

Possible Impact To Clustering Factor Now ROWIDs Are Updated When Rows Migrate Part II (“Dancing Out In Space”) March 7, 2023

Posted by Richard Foote in 19c, 19c New Features, Attribute Clustering, Autonomous Data Warehouse, Autonomous Database, Autonomous Transaction Processing, CBO, Changing ROWID, Clustering Factor, Data Clustering, David Bowie, Full Table Scans, Index Access Path, Index Internals, Index Rebuild, Index statistics, Leaf Blocks, Migrated Rows, Oracle, Oracle 21c, Oracle Blog, Oracle Cloud, Oracle Cost Based Optimizer, Oracle General, Oracle Indexes, Oracle Statistics, Oracle19c, Performance Tuning, Richard's Musings, ROWID.
1 comment so far

In my previous post, I discussed how the clustering of data can be impacted if rows migrate and how this in turn can have a detrimental impact on the efficiency of associated indexes.

In this post, I’ll discuss what you can do (and not do) to remedy things in the relatively unlikely event that you hit this issue with migrated rows.

I’ll just discuss initially the example where the table is defined without ENABLE ROW MOVEMENT enabled in the Transaction Processing Autonomous Database (and so does NOT update ROWIDs on the fly when a row migrates).

I’ll start by again creating and populating a tightly packed table, with the data inserted in ID column order:

SQL> create table bowie(id number, code1 number, code2 number, code3 number, code4 number, code5 number, code6 number, code7 number, code8 number, code9 number, code10 number, code11 number, code12 number, code13 number, code14 number, code15 number, code16 number, code17 number, code18 number, code19 number, code20 number, name varchar2(142)) PCTFREE 0;

Table BOWIE created.

SQL> insert into bowie SELECT rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, 'BOWIE' FROM dual CONNECT BY LEVEL <= 200000;

200,000 rows inserted.

SQL> commit;

Commit complete.

I’ll now create an index on this well ordered/clustered ID column:

SQL> create index bowie_id_i on bowie(id);

Index BOWIE_ID_I created.

Next, I’ll update the table, increasing the size of the rows such that I generate a bunch of migrated rows:

SQL> update bowie set name='THE RISE AND FALL OF BOWIE STARDUST AND THE SPIDERS FROM MARS';

200,000 rows updated.

SQL> commit;

Commit complete.

 

If we check the number of migrated rows:

SQL> analyze table bowie compute statistics;

Table BOWIE analyzed.

SQL> select table_name, num_rows, blocks, empty_blocks, avg_space, avg_row_len, chain_cnt from user_tables

where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS    EMPTY_BLOCKS    AVG_SPACE    AVG_ROW_LEN    CHAIN_CNT
_____________ ___________ _________ _______________ ____________ ______________ ____________
BOWIE              200000      4906              86          414            170        56186

 

We notice there are indeed 56186 migrated rows.

If we check the current Clustering Factor of the index:

SQL> execute dbms_stats.delete_table_stats(ownname=>null, tabname=>'BOWIE');

PL/SQL procedure successfully completed.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

SQL> select table_name, num_rows, blocks from user_tables where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS
_____________ ___________ _________
BOWIE              200000      4906

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE';

   INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
_____________ _________ ______________ ____________________
BOWIE_ID_I            1            473                 3250

 

We notice the index still has an excellent Clustering Factor of just 3250. As the ROWIDs are NOT updated in this example when rows migrate, the index retains the same Clustering Factor as before the Update statement.

If we run the following query that returns 4200 rows (as per my previous post):

SQL> select * from bowie where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
_______________________________________________________________________________________________________________
SQL_ID c376kdhy5b0x9, child number 0
-------------------------------------
select * from bowie where id between 1 and 4200

Plan hash value: 1405654398

------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows | A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |   4200 |00:00:00.01 |    2771 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   4200 |00:00:00.01 |    2771 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      11 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)


Statistics
-----------------------------------------------------------
          2 CPU used by this session
          2 CPU used when call started
          3 DB time
      24901 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
       2762 buffer is not pinned count
       7005 buffer is pinned count
        324 bytes received via SQL*Net from client
     461909 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
       2771 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
       2771 consistent gets from cache
       2770 consistent gets pin
       2770 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
   22700032 logical read bytes from cache
       2770 no work - consistent read gets
         73 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          1 session cursor cache count
          1 session cursor cache hits
       2771 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4200 table fetch by rowid
       1366 table fetch continued row
          3 user calls

We can see the query currently uses 2771 consistent gets, which is significantly higher than it could be, as Oracle has to visit the original table block and then follow the pointer to the new location for any migrated row that needs to be retrieved.

However, if we look at the cost of the current plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'c376kdhy5b0x9',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
____________________________________________________________________________________________________________________________________
SQL_ID c376kdhy5b0x9, child number 0

-------------------------------------

select * from bowie where id between 1 and 4200

Plan hash value: 1405654398

---------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
---------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |       |    80 (100)|   4200 |00:00:00.01 |    2771 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   684K|      80 (0)|   4200 |00:00:00.01 |    2771 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |       |      11 (0)|   4200 |00:00:00.01 |      11 |
---------------------------------------------------------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________
Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

 

We can see it only has a cost of 80, as Oracle does not consider the additional accesses required now for these migrated rows. With such a perfect Clustering Factor, this cost is not particularly accurate and does not represent the true cost of the 2771 consistent gets now required.

Now there are various ways we can look at fixing this issue with all these migrated rows requiring additional consistent gets to access.

One method is to capture all the ROWIDs of the migrated rows, copy these rows to a temporary holding table, delete these rows and then re-insert them all back into the table from the temporary table.

We can identify the migrated rows by creating the CHAIN_ROWS table as per the Oracle supplied UTLCHAIN.SQL script and then use the ANALYZE command to store their ROWIDs in this CHAIN_ROWS table:

SQL> create table CHAINED_ROWS (
2 owner_name varchar2(128),
3 table_name varchar2(128),
4 cluster_name varchar2(128),
5 partition_name varchar2(128),
6 subpartition_name varchar2(128),
7 head_rowid rowid,
8 analyze_timestamp date
9* );

Table CHAINED_ROWS created.

SQL> analyze table bowie list chained rows;

Table BOWIE analyzed.

SQL> select table_name, head_rowid from chained_rows where table_name='BOWIE' and rownum<=10;

   TABLE_NAME            HEAD_ROWID
_____________ _____________________
BOWIE         AAAqFjAAAAAE6CzAAP
BOWIE         AAAqFjAAAAAE6CzAAR
BOWIE         AAAqFjAAAAAE6CzAAU
BOWIE         AAAqFjAAAAAE6CzAAW
BOWIE         AAAqFjAAAAAE6CzAAZ
BOWIE         AAAqFjAAAAAE6CzAAb
BOWIE         AAAqFjAAAAAE6CzAAe
BOWIE         AAAqFjAAAAAE6CzAAg
BOWIE         AAAqFjAAAAAE6CzAAj
BOWIE         AAAqFjAAAAAE6CzAAl

 

Another method we can now utilise is to simply MOVE ONLINE the table:

SQL> alter table bowie move online;

Table BOWIE altered.

 

If we now look at the number of migrated rows after the table reorg:

SQL> analyze table bowie compute statistics;

Table BOWIE analyzed.

SQL> select table_name, num_rows, blocks, empty_blocks, avg_space, avg_row_len, chain_cnt from user_tables where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS    EMPTY_BLOCKS    AVG_SPACE    AVG_ROW_LEN    CHAIN_CNT
_____________ ___________ _________ _______________ ____________ ______________ ____________
BOWIE              200000      4936              56          838            169            0

 

We can see we no longer have any migrated rows.

BUT, if we now look at the Clustering Factor of this index:

SQL> execute dbms_stats.delete_table_stats(ownname=>null, tabname=>'BOWIE');

PL/SQL procedure successfully completed.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

SQL> select table_name, num_rows, blocks from user_tables where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS
_____________ ___________ _________
BOWIE              200000      4936

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE';

   INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
_____________ _________ ______________ ____________________
BOWIE_ID_I            1            473               114560

 

We can see it has now significantly increased to 114560 (previously it was just 3250).

The problem of course is that if the ROWIDs now represent the correct new physical location of the migrated rows, the previously perfect clustering/ordering of the ID column has been impacted.

If we now re-run the query returning the 4200 rows:

SQL> select * from bowie where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________
SQL_ID c376kdhy5b0x9, child number 0
-------------------------------------
select * from bowie where id between 1 and 4200

Plan hash value: 1845943507

---------------------------------------------------------------------------------------------
| Id | Operation                  | Name  | Starts | E-Rows | A-Rows | A-Time     | Buffers |
---------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT           |       |      1 |        |   4200 |00:00:00.02 |    4857 |
|* 1 |  TABLE ACCESS STORAGE FULL | BOWIE |      1 |   4200 |   4200 |00:00:00.02 |    4857 |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - storage(("ID"<=4200 AND "ID">=1))
       filter(("ID"<=4200 AND "ID">=1))

Statistics
-----------------------------------------------------------
          3 CPU used by this session
          3 CPU used when call started
       4849 Cached Commit SCN referenced
          2 DB time
      25870 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
          2 buffer is not pinned count
        324 bytes received via SQL*Net from client
     461962 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          9 calls to kcmgcs
       4857 consistent gets
       4857 consistent gets from cache
       4857 consistent gets pin
       4857 consistent gets pin (fastpath)
          2 execute count
   39788544 logical read bytes from cache
       4850 no work - consistent read gets
         72 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          2 process last non-idle time
          1 session cursor cache count
       4857 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4850 table scan blocks gotten
     200000 table scan disk non-IMC rows gotten
     200000 table scan rows gotten
          1 table scans (short tables)
          3 user calls

 

Oracle is now performing a Full Table Scan (FTS). The number of consistent gets now at 4857 is actually worse than when we had the migrated rows (previously at 2771)

The Clustering Factor of the ID column is now so bad, that returning 4200 rows via such an index is just too expensive. The FTS is now deemed the cheaper option by the CBO.

If we look at the CBO cost of using this FTS plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'c376kdhy5b0x9',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________________________
SQL_ID c376kdhy5b0x9, child number 0
-------------------------------------
select * from bowie where id between 1 and 4200

Plan hash value: 1845943507

------------------------------------------------------------------------------------------------------------------
| Id | Operation                  | Name  | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT           |       |      1 |        |       |  1340 (100)|   4200 |00:00:00.02 |    4857 |
|* 1 |  TABLE ACCESS STORAGE FULL | BOWIE |      1 |   4200 |   684K|    1340 (1)|   4200 |00:00:00.02 |    4857 |
------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - storage(("ID"<=4200 AND "ID">=1))
       filter(("ID"<=4200 AND "ID">=1))

 

We can see the cost of this plan is 1340.

If we compare this with the cost of using the (now deemed) inefficient index:

SQL> select /*+ index (bowie) */ * from bowie where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
_______________________________________________________________________________________________________________
SQL_ID 9215hkzd3v1up, child number 0
-------------------------------------
select /*+ index (bowie) */ * from bowie where id between 1 and 4200

Plan hash value: 1405654398

------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows | A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |   4200 |00:00:00.01 |    2784 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   4200 |00:00:00.01 |    2784 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      11 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)


Statistics
-----------------------------------------------------------
          2 CPU used by this session
          2 CPU used when call started
       2741 Cached Commit SCN referenced
          2 DB time
      12633 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
       2775 buffer is not pinned count
       5626 buffer is pinned count
        345 bytes received via SQL*Net from client
     462170 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
       2784 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
       2784 consistent gets from cache
       2783 consistent gets pin
       2783 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
   22806528 logical read bytes from cache
       2783 no work - consistent read gets
         72 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          4 process last non-idle time
          1 session cursor cache count
          1 session cursor cache hits
       2784 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4200 table fetch by rowid
          3 user calls

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'9215hkzd3v1up',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
____________________________________________________________________________________________________________________________________
SQL_ID 9215hkzd3v1up, child number 0

-------------------------------------

select /*+ index (bowie) */ * from bowie where id between 1 and 4200

Plan hash value: 1405654398

---------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
---------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |       |  2418 (100)|   4200 |00:00:00.01 |    2784 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   684K|    2418 (1)|   4200 |00:00:00.01 |    2784 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |       |      11 (0)|   4200 |00:00:00.01 |      11 |
---------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

 

We can see the CBO cost of the index is now 2418, more than the 1340 cost of using the FTS.

So in the scenario where by migrating a significant number of rows, we impact the Clustering Factor and so the efficiency of vital indexes in our applications, we need to eliminate the migrated rows in a more thoughtful manner.

An option we have available is to first add an appropriate Clustering Attribute before we perform the table reorg:

SQL> alter table bowie add clustering by linear order (id);

Table BOWIE altered.

SQL> alter table bowie move online;

Table BOWIE altered.

 

If we now look at the Clustering Factor of this important index:

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

SQL> select table_name, num_rows, blocks from user_tables where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS
_____________ ___________ _________
BOWIE              200000      4936

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE';

   INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
_____________ _________ ______________ ____________________
BOWIE_ID_I            1            473                 4850

 

The Clustering Factor has been reduced down to the almost perfect 4850, down from the previous 114560.

If we now re-run the query:

SQL> select * from bowie where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
_______________________________________________________________________________________________________________
SQL_ID c376kdhy5b0x9, child number 0
-------------------------------------
select * from bowie where id between 1 and 4200

Plan hash value: 1405654398

------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows | A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |   4200 |00:00:00.01 |     102 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   4200 |00:00:00.01 |     102 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      11 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)


Statistics
-----------------------------------------------------------
          1 CPU used by this session
          1 CPU used when call started
         89 Cached Commit SCN referenced
          1 DB time
      11249 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
         93 buffer is not pinned count
       8308 buffer is pinned count
        324 bytes received via SQL*Net from client
     462165 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
        102 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
        102 consistent gets from cache
        101 consistent gets pin
        101 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
     835584 logical read bytes from cache
        101 no work - consistent read gets
         72 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          1 session cursor cache count
          1 session cursor cache hits
        102 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4200 table fetch by rowid
          3 user calls

We can see the query now automatically uses the index and only requires just 102 consistent gets (down from 4857 when it performed the FTS and down from 2771 when we had the migrated rows).

If we look at the cost of this new plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'c376kdhy5b0x9',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
____________________________________________________________________________________________________________________________________
SQL_ID c376kdhy5b0x9, child number 0

-------------------------------------

select * from bowie where id between 1 and 4200

Plan hash value: 1405654398

---------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
---------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |       |   113 (100)|   4200 |00:00:00.01 |     102 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   684K|     113 (0)|   4200 |00:00:00.01 |     102 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |       |      11 (0)|   4200 |00:00:00.01 |      11 |
---------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

 

We can see the plan has a cost of just 113, which is both much more accurate and close to the 102 consistent gets and much less than the previous cost of 1340 for the FTS plan.

So in specific scenarios where by having migrated rows we significantly impact the Clustering Factor of indexes important to our applications, we have to be a little cleverer in how we address the migrated rows.

This can also the case in the new scenario where Oracle automatically updates the ROWIDs of migrated rows, as I’ll discuss in my next post…

Possible Impact To Clustering Factor Now ROWIDs Are Updated When Rows Migrate Part I (“Growin’ Up”) March 1, 2023

Posted by Richard Foote in 19c, 19c New Features, Attribute Clustering, Autonomous Data Warehouse, Autonomous Database, Autonomous Transaction Processing, BLEVEL, CBO, Changing ROWID, Clustering Factor, Data Clustering, Hints, Index Access Path, Index Block Splits, Index Delete Operations, Index Height, Index Internals, Index Rebuild, Index statistics, Leaf Blocks, Migrated Rows, Oracle, Oracle Blog, Oracle Cloud, Oracle Cost Based Optimizer, Oracle General, Oracle Indexes, Oracle Indexing Internals Webinar, Oracle Statistics, Oracle19c, Performance Tuning, Richard Foote Training, Richard's Blog, ROWID.
2 comments

In my previous post I discussed how an index can potentially be somewhat inflated in size after ROWIDs are updated on the fly after a substantial number of rows are migrated.

However, there’s another key “factor” of an index that in some scenarios can be impacted by this new ROWID behaviour with regard migrated rows.

To highlight this scenario, I’ll again start by creating and populating a table with ENABLE ROW MOVEMENT disabled:

SQL> create table bowie(id number, code1 number, code2 number, code3 number, code4 number, code5 number, code6 number, code7 number, code8 number, code9 number, code10 number, code11 number, code12 number, code13 number, code14 number, code15 number, code16 number, code17 number, code18 number, code19 number, code20 number, name varchar2(142)) PCTFREE 0;

Table BOWIE created.

SQL> insert into bowie SELECT rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, 'BOWIE' FROM dual CONNECT BY LEVEL <= 200000;

200,000 rows inserted.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

I’ll next create an index on the ID column. The important aspect with the ID column is that the data is entered monotonically in ID column order, so the associated index will have an excellent (very low) Clustering Factor:

SQL> create index bowie_id_i on bowie(id);

Index BOWIE_ID_I created.

If we look at some key statistics of the table and index:

SQL> select table_name, num_rows, blocks from user_tables where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS
_____________ ___________ _________
BOWIE              200000      3268

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE';

   INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
_____________ _________ ______________ ____________________
BOWIE_ID_I            1            473                 3250

We can see that the number of table blocks is 3268, the number of index leaf blocks is 473 and we indeed have a near perfect Clustering Factor of 3250.

If we run a couple of queries:

SQL> select * from bowie where id between 1 and 1000;

1,000 rows selected.

PLAN_TABLE_OUTPUT
_______________________________________________________________________________________________________________
SQL_ID gz5u92hmjwz1h, child number 0
-------------------------------------
select * from bowie where id between 1 and 1000

Plan hash value: 1405654398

------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows | A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |   1000 |00:00:00.01 |      18 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   1000 |   1000 |00:00:00.01 |      18 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   1000 |   1000 |00:00:00.01 |       4 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=1000)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          1 CPU used by this session
          1 CPU used when call started
       7353 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
         16 buffer is not pinned count
       1985 buffer is pinned count
        324 bytes received via SQL*Net from client
     171305 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
         18 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
         18 consistent gets from cache
         17 consistent gets pin
         17 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
     147456 logical read bytes from cache
         17 no work - consistent read gets
         38 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          2 session cursor cache count
         18 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       1000 table fetch by rowid
          3 user calls

We can see for this first query that returns 1000 rows, it requires just 18 consistent gets, thanks primarily due to the efficient index with the perfect Clustering Factor.

If we look at the cost of this plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'gz5u92hmjwz1h',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
____________________________________________________________________________________________________________________________________
SQL_ID gz5u92hmjwz1h, child number 0

-------------------------------------

select * from bowie where id between 1 and 1000

Plan hash value: 1405654398

---------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
---------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |       |    21 (100)|   1000 |00:00:00.01 |      18 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   1000 |   108K|      21 (0)|   1000 |00:00:00.01 |      18 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   1000 |       |       4 (0)|   1000 |00:00:00.01 |       4 |
---------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=1000)

We can see the plan has an accurate cost of just 21.

If we now run a similar query that returns a few more rows:

SQL> select * from bowie where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
_______________________________________________________________________________________________________________
SQL_ID c376kdhy5b0x9, child number 0
-------------------------------------
select * from bowie where id between 1 and 4200

Plan hash value: 1405654398

------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows | A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |   4200 |00:00:00.01 |      68 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   4200 |00:00:00.01 |      68 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      11 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          1 CPU used by this session
          1 CPU used when call started
          1 DB time
      11353 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
         59 buffer is not pinned count
       8342 buffer is pinned count
        324 bytes received via SQL*Net from client
     461834 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
         68 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
         68 consistent gets from cache
         67 consistent gets pin
         67 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
     557056 logical read bytes from cache
         67 no work - consistent read gets
         73 non-idle wait count
         2 opened cursors cumulative
         1 opened cursors current
         2 parse count (total)
         1 process last non-idle time
         2 session cursor cache count
        68 session logical reads
         1 sorts (memory)
      2024 sorts (rows)
      4200 table fetch by rowid
         3 user calls

We can see that it only required just 68 consistent gets to return 4200 rows, thanks to the excellent data clustering and associated very low Clustering Factor.

If we look at the cost of this plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'c376kdhy5b0x9',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
____________________________________________________________________________________________________________________________________
SQL_ID c376kdhy5b0x9, child number 0

-------------------------------------

select * from bowie where id between 1 and 4200

Plan hash value: 1405654398

---------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
---------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |       |    80 (100)|   4200 |00:00:00.01 |      68 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   455K|      80 (0)|   4200 |00:00:00.01 |      68 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |       |      11 (0)|   4200 |00:00:00.01 |      11 |
---------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

 

We can see the cost of the plan is currently a relatively accurate 80.

OK, let’s now perform an update on this table that generates a bunch of migrated rows:

SQL> update bowie set name='THE RISE AND FALL OF BOWIE STARDUST AND THE SPIDERS FROM MARS';

200,000 rows updated.

SQL> commit;

Commit complete.

If we now look at the table and index statistics:

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

SQL> select table_name, num_rows, blocks from user_tables where table_name='BOWIE';

   TABLE_NAME    NUM_ROWS    BLOCKS
_____________ ___________ _________
BOWIE              200000      4906

We can see that the table blocks value has increased to 4906 (previously 3268). This as explained previously is to due in large part to the increased NAME column values and also due to the pointers in the original table blocks that point to the new locations of the migrated rows.

This relates to approximately a 50% increase in table blocks.

If we look at the current index statistics:

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE';

   INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
_____________ _________ ______________ ____________________
BOWIE_ID_I            1            473                 3250

We can see that these values are all unchanged, as the ROWIDs in indexes remain unchanged when a row migrates, when ENABLE ROW MOVEMENT is not set.

Therefore, when we re-run these same queries:

SQL> select * from bowie where id between 1 and 1000;

1,000 rows selected.

PLAN_TABLE_OUTPUT
_______________________________________________________________________________________________________________
SQL_ID gz5u92hmjwz1h, child number 0
-------------------------------------
select * from bowie where id between 1 and 1000

Plan hash value: 1405654398

------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows | A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |   1000 |00:00:00.01 |     666 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   1000 |   1000 |00:00:00.01 |     666 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   1000 |   1000 |00:00:00.01 |       4 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=1000)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          1 DB time
       7967 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
        664 buffer is not pinned count
       1664 buffer is pinned count
        324 bytes received via SQL*Net from client
     171419 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
        666 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
        666 consistent gets from cache
        665 consistent gets pin
        665 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
    5455872 logical read bytes from cache
        665 no work - consistent read gets
         37 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          2 session cursor cache count
        666 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       1000 table fetch by rowid
        327 table fetch continued row
          3 user calls

The number of consistent gets has increased significantly to 666 (previously it was just 18).

Now we can attributed an increase of approximately 50% of the previous consistent gets (18 x 0.50 = 9) due to the 50% increase in table blocks required now to store the rows due to the increased row size.

We can also attribute an additional 327 consistent gets for the table fetch continued row value listed in the statistics, representing the extra consistent gets required to access the migrated rows from their new physical location.

But 18 + 9 + 327 = 354 still leaves us short of the new 666 consistent gets value.

The problem with having to visit another table block to get a row from its new location is that it means Oracle has to re-access again the original table block to get the next row (rather than reading multiple rows with the same consistent get).

So it’s actually approximately 2 x table fetch continued row, by which the number of consistent gets is going to increase when accessing migrated rows (noting that the last migrated row in a block will only incur a additional consistent get as the next table block accessed will differ regardless).

If we look at the new CBO cost for this plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'gz5u92hmjwz1h',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
____________________________________________________________________________________________________________________________________
SQL_ID gz5u92hmjwz1h, child number 0

-------------------------------------

select * from bowie where id between 1 and 1000

Plan hash value: 1405654398

---------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
---------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |       |    21 (100)|   1000 |00:00:00.01 |     666 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   1000 |   163K|      21 (0)|   1000 |00:00:00.01 |     666 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   1000 |       |       4 (0)|   1000 |00:00:00.01 |       4 |
---------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=1000)

 

We notice the CBO cost for this plan remains unchanged at 21.

This is totally to be expected, as the index statistics by which the cost of an index scan is calculated are unchanged.

Considering the rough “rule of thumb” is that the CBO cost of an index scan should be in the ball-park of the number of possible IOs, the fact the plan now uses 666 consistent gets highlights this cost of just 21 is no longer as accurate…

If we look at the second SQL that returns 4200 rows:

SQL> select * from bowie where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
_______________________________________________________________________________________________________________
SQL_ID c376kdhy5b0x9, child number 0
-------------------------------------
select * from bowie where id between 1 and 4200

Plan hash value: 1405654398

------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows | A-Rows | A-Time     | Buffers |
------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |   4200 |00:00:00.01 |    2771 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   4200 |00:00:00.01 |    2771 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      11 |
------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          2 CPU used by this session
          2 CPU used when call started
          2 DB time
      14103 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
       2762 buffer is not pinned count
       7005 buffer is pinned count
        324 bytes received via SQL*Net from client
      461947 bytes sent via SQL*Net to client
           2 calls to get snapshot scn: kcmgss
           2 calls to kcmgcs
        2771 consistent gets
           1 consistent gets examination
           1 consistent gets examination (fastpath)
        2771 consistent gets from cache
        2770 consistent gets pin
        2770 consistent gets pin (fastpath)
           2 execute count
           1 index range scans
    22700032 logical read bytes from cache
        2770 no work - consistent read gets
          72 non-idle wait count
           2 opened cursors cumulative
           1 opened cursors current
           2 parse count (total)
           1 process last non-idle time
           2 session cursor cache count
        2771 session logical reads
           1 sorts (memory)
        2024 sorts (rows)
        4200 table fetch by rowid
        1366 table fetch continued row
           3 user calls

We again notice consistent gets has increased significantly to 2771 (previously it was just 68). Again, these additional consistent gets can not be attributed to the extra size of the table and the additional approximate 2 x 1366 table fetch continued row gets.

If we now look at the cost of this plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'c376kdhy5b0x9',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
________________________________________________________________________________________________________________________
____________

SQL_ID c376kdhy5b0x9, child number 0

-------------------------------------

select * from bowie where id between 1 and 4200

Plan hash value: 1405654398

---------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name       | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
---------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |            |      1 |        |       |    80 (100)|   4200 |00:00:00.01 |    2771 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE      |      1 |   4200 |   684K|      80 (0)|   4200 |00:00:00.01 |    2771 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE_ID_I |      1 |   4200 |       |      11 (0)|   4200 |00:00:00.01 |      11 |
---------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

 

We again notice the CBO cost for this plan remains unchanged at 80, again totally expected as the underlying index statistics have remain unchanged after the update statement.

But again, not necessary as accurate a cost as it was previously…

 

If we repeat this demo, but this time on a table with ENABLE ROW MOVEMENT enabled:

SQL> create table bowie2(id number, code1 number, code2 number, code3 number, code4 number, code5 number, code6 number, code7 number, code8 number, code9 number, code10 number, code11 number, code12 number, code13 number, code14 number, code15 number, code16 number, code17 number, code18 number, code19 number, code20 number, name varchar2(142)) PCTFREE 0 ENABLE ROW MOVEMENT;

Table BOWIE2 created.

SQL> insert into bowie2 SELECT rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, rownum, 'BOWIE' FROM dual CONNECT BY LEVEL <= 200000;

200,000 rows inserted.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE2', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

SQL> create index bowie2_id_i on bowie2(id);

Index BOWIE2_ID_I created.

SQL> select table_name, num_rows, blocks from user_tables where table_name='BOWIE2';

   TABLE_NAME    NUM_ROWS    BLOCKS
_____________ ___________ _________
BOWIE2             200000      3268

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE2';

        INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
__________________ _________ ______________ ____________________
BOWIE2_ID_I                1            473                 3250

 

The table and index statistics are currently identical to the previous demo.

If we run the same two equivalent queries:

 

SQL> select * from bowie2 where id between 1 and 1000;

1,000 rows selected.

PLAN_TABLE_OUTPUT
________________________________________________________________________________________________________________
SQL_ID gtkw2704bxj7q, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 1000

Plan hash value: 3243780227

-------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows | A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |   1000 |00:00:00.01 |      18 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   1000 |   1000 |00:00:00.01 |      18 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   1000 |   1000 |00:00:00.01 |       4 |
-------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=1000)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          1 CPU used by this session
          1 CPU used when call started
       7909 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
         16 buffer is not pinned count
       1985 buffer is pinned count
        325 bytes received via SQL*Net from client
     171306 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
         18 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
         18 consistent gets from cache
         17 consistent gets pin
         17 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
     147456 logical read bytes from cache
         17 no work - consistent read gets
         37 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          2 session cursor cache count
         18 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       1000 table fetch by rowid
     3 user calls

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'gtkw2704bxj7q',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________________________________________
SQL_ID gtkw2704bxj7q, child number 0

-------------------------------------

select * from bowie2 where id between 1 and 1000

Plan hash value: 3243780227

----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |       |    21 (100)|   1000 |00:00:00.01 |      18 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   1000 |   108K|      21 (0)|   1000 |00:00:00.01 |      18 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   1000 |       |       4 (0)|   1000 |00:00:00.01 |       4 |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=1000)



SQL> select * from bowie2 where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
________________________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

-------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows | A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |   4200 |00:00:00.01 |      68 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   4200 |00:00:00.01 |      68 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      11 |
-------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          1 CPU used by this session
          1 CPU used when call started
          2 DB time
      13157 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
         59 buffer is not pinned count
       8342 buffer is pinned count
        325 bytes received via SQL*Net from client
     461838 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
         68 consistent gets
          1 consistent gets examination
          1 consistent gets examination (fastpath)
         68 consistent gets from cache
         67 consistent gets pin
         67 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
     557056 logical read bytes from cache
         67 no work - consistent read gets
         73 non-idle wait count
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          2 session cursor cache count
         68 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       4200 table fetch by rowid
          3 user calls

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'25qktyn35b662',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0

-------------------------------------

select * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |       |    80 (100)|   4200 |00:00:00.01 |      68 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   455K|      80 (0)|   4200 |00:00:00.01 |      68 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |       |      11 (0)|   4200 |00:00:00.01 |      11 |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

 

With identical table/index statistics, we notice as expected that both SQLs have the same consistent gets and CBO costs as with the previous demo.

If we now repeat the equivalent Update statement:

SQL> update bowie2 set name='THE RISE AND FALL OF BOWIE STARDUST AND THE SPIDERS FROM MARS';

200,000 rows updated.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'BOWIE2', estimate_percent=> null, no_invalidate=>false);

PL/SQL procedure successfully completed.

 

If we look at the table statistics:

SQL> select table_name, num_rows, blocks from user_tables where table_name='BOWIE2';

   TABLE_NAME   NUM_ROWS     BLOCKS
_____________ ___________ _________
BOWIE2             200000      4654

 

We notice the number of table blocks has increased to 4654 due to the increased row lengths, but not as much as with the previous demo (where table blocks increased to 4906) as in this scenario, Oracle does not have to store the row location pointers in the original blocks for the migrated rows.

If we look at the index statistics:

SQL> select index_name, blevel, leaf_blocks, clustering_factor from user_indexes where table_name='BOWIE2';

    INDEX_NAME    BLEVEL    LEAF_BLOCKS    CLUSTERING_FACTOR
______________ _________ ______________ ____________________
BOWIE2_ID_I            2            945               109061

We notice that these are substantially different from the first demo, where ROWIDs for migrated rows are not updated on the fly.

By now updating the ROWIDs, the indexes can possibly increase in size as they have to store both the previous and new ROWIDs in separate index entries and hence Oracle is more likely to perform additional index block splits (as I discussed in my previous post).

The LEAF_BLOCKS are now 945 (previously 473) and even the BLEVEL has increased from 1 to 2.

Additionally, and perhaps importantly for specific key indexes, the Clustering Factor value of indexes can also be impacted. By migrating rows and physically storing them in different locations, this can potentially detrimentally impact the tight clustering of rows based on specific column values.

The Clustering Factor for the index on the monotonically increased ID column has now increased significantly to 109061, up from the previously perfect 3250.

So columns that have naturally good clustering (e.g.: monotonically increasing values such as IDs and dates) or have been manually well clustered for performance purposes, can have the Clustering Factor of associated indexes detrimentally impacted by migrated rows.

If we re-run the first query:

SQL> select * from bowie2 where id between 1 and 1000;

1,000 rows selected.

PLAN_TABLE_OUTPUT
________________________________________________________________________________________________________________
SQL_ID gtkw2704bxj7q, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 1000

Plan hash value: 3243780227

-------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows | A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |   1000 |00:00:00.01 |     639 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   1000 |   1000 |00:00:00.01 |     639 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   1000 |   1000 |00:00:00.01 |       7 |
-------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=1000)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          1 CPU used by this session
          1 CPU used when call started
          1 DB time
      15262 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
        634 buffer is not pinned count
       1367 buffer is pinned count
        325 bytes received via SQL*Net from client
     171421 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
        639 consistent gets
          2 consistent gets examination
          2 consistent gets examination (fastpath)
        639 consistent gets from cache
        637 consistent gets pin
        637 consistent gets pin (fastpath)
          2 execute count
          1 index range scans
    5234688 logical read bytes from cache
        637 no work - consistent read gets
         38 non-idle wait count
          1 non-idle wait time
          2 opened cursors cumulative
          1 opened cursors current
          2 parse count (total)
          1 process last non-idle time
          2 session cursor cache count
        639 session logical reads
          1 sorts (memory)
       2024 sorts (rows)
       1000 table fetch by rowid
          3 user calls

I discussed in a previous post how by updating the ROWIDs of migrated rows we can improve performance, as Oracle can go directly to the correct new physical location of a migrated row.

But for some specific indexes, where data clustering is crucial, and we have a significant number migrated rows, this might not necessarily be the case.

We can see consistent gets here has increased to 639 (previously is was just 21), and so not hugely different from the 666 consistent gets required to fetch the migrated rows when the ROWIDs were not updated in the first demo.

If we look at the CBO costings:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'gtkw2704bxj7q',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________________________________________
SQL_ID gtkw2704bxj7q, child number 0

-------------------------------------

select * from bowie2 where id between 1 and 1000

Plan hash value: 3243780227

----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |       |   553 (100)|   1000 |00:00:00.01 |     639 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   1000 |   163K|     553 (0)|   1000 |00:00:00.01 |     639 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   1000 |       |       7 (0)|   1000 |00:00:00.01 |       7 |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=1000)

 

We can see the CBO cost has increased significantly to 553 (previously it was just 21).

With a much increased Clustering Factor, this will obviously impact the CBO costs of associated index scans.

In very extreme cases, these possible changes in the Clustering Factor can even impact the viability of using the index.

If we re-run the second query returning the 4200 rows:

SQL> select * from bowie2 where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 4200

Plan hash value: 1495904576

----------------------------------------------------------------------------------------------
| Id | Operation                  | Name   | Starts | E-Rows | A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT           |        |      1 |        |   4200 |00:00:00.02 |    4572 |
|* 1 |  TABLE ACCESS STORAGE FULL | BOWIE2 |      1 |   4200 |   4200 |00:00:00.02 |    4572 |
----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - storage(("ID"<=4200 AND "ID">=1))
       filter(("ID"<=4200 AND "ID">=1))

We can see that the CBO has now chosen to perform a Full Table Scan (FTS), rather than use the now less efficient index to return this number of rows.

If we look at the CBO costings of this FTS plan:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'25qktyn35b662',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
______________________________________________________________________________________________________________________
SQL_ID 25qktyn35b662, child number 0
-------------------------------------
select * from bowie2 where id between 1 and 4200

Plan hash value: 1495904576

-------------------------------------------------------------------------------------------------------------------
| Id | Operation                  | Name   | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT           |        |      1 |        |       |  1264 (100)|   4200 |00:00:00.02 |    4572 |
|* 1 |  TABLE ACCESS STORAGE FULL | BOWIE2 |      1 |   4200 |   684K|    1264 (1)|   4200 |00:00:00.02 |    4572 |
-------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - storage(("ID"<=4200 AND "ID">=1))
       filter(("ID"<=4200 AND "ID">=1))

 

The cost of the FTS plan is 1264.

If we compare this is a plan that used the index:

SQL> select /*+ index (bowie2) */ * from bowie2 where id between 1 and 4200;

4,200 rows selected.

PLAN_TABLE_OUTPUT
________________________________________________________________________________________________________________
SQL_ID bzm2vhchqpq7w, child number 0
-------------------------------------
select /*+ index (bowie2) */ * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

-------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows | A-Rows | A-Time     | Buffers |
-------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |   4200 |00:00:00.01 |    2665 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   4200 |00:00:00.01 |    2665 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |   4200 |00:00:00.01 |      21 |
-------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID">=1 AND "ID"<=4200)

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 1 because of no expensive parallel operation

Statistics
-----------------------------------------------------------
          2 CPU used by this session
          2 CPU used when call started
          2 DB time
      14531 RM usage
          3 Requests to/from client
          2 SQL*Net roundtrips to/from client
       2646 buffer is not pinned count
       5755 buffer is pinned count
        348 bytes received via SQL*Net from client
     462143 bytes sent via SQL*Net to client
          2 calls to get snapshot scn: kcmgss
          2 calls to kcmgcs
       2665 consistent gets
         2 consistent gets examination
         2 consistent gets examination (fastpath)
      2665 consistent gets from cache
      2663 consistent gets pin
      2663 consistent gets pin (fastpath)
         2 execute count
         1 index range scans
  21831680 logical read bytes from cache
      2663 no work - consistent read gets
        73 non-idle wait count
         2 opened cursors cumulative
         1 opened cursors current
         2 parse count (total)
         3 process last non-idle time
         2 session cursor cache count
      2665 session logical reads
         1 sorts (memory)
      2024 sorts (rows)
      4200 table fetch by rowid
         3 user calls

SQL> SELECT * FROM TABLE(DBMS_XPLAN.display_cursor(sql_id=>'bzm2vhchqpq7w',format=>'ALLSTATS LAST +cost +bytes'));

PLAN_TABLE_OUTPUT
_____________________________________________________________________________________________________________________________________
SQL_ID bzm2vhchqpq7w, child number 0

-------------------------------------

select /*+ index (bowie2) */ * from bowie2 where id between 1 and 4200

Plan hash value: 3243780227

----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation                            | Name        | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time     | Buffers |
----------------------------------------------------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT                     |             |      1 |        |       |  2314 (100)|   4200 |00:00:00.01 |    2665 |
|  1 |  TABLE ACCESS BY INDEX ROWID BATCHED | BOWIE2      |      1 |   4200 |   684K|    2314 (1)|   4200 |00:00:00.01 |    2665 |
|* 2 |   INDEX RANGE SCAN                   | BOWIE2_ID_I |      1 |   4200 |       |      22 (0)|   4200 |00:00:00.01 |      21 |
----------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

2 - access("ID">=1 AND "ID"<=4200)

 

The cost of using the index to retrieve the 4200 rows is 2310, more than the 1264 of the FTS.

 

For the vast majority of indexes, updating the ROWIDs for migrated rows will result in better performance, as such indexes will be able to directly access the correct new physical location of migrated rows, rather than having to visit the original table block and then follow the stored pointer to the new table block.

But for some very specific indexes, where data clustering is crucial, AND we have a significant number migrated rows, this might not necessarily be the case. The performance benefit might be minimal at best.

That’s more than enough for one post 🙂

In my next post, I’ll discuss how to potentially remedy these performance implications, both for tables with or without ENABLE TABLE MOVEMENT enabled…