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Oracle 19c Automatic Indexing: Indexing With Stale Statistics Part I (Dead Against It) October 6, 2020

Posted by Richard Foote in 19c, 19c New Features, Automatic Indexing, Autonomous Data Warehouse, Autonomous Database, Autonomous Transaction Processing, CBO, Exadata, Exadata X8, Full Table Scans, High Frequency Statistics Collection, Index Access Path, Index statistics, Oracle, Oracle Cloud, Oracle Cost Based Optimizer, Oracle General, Oracle Indexes, Performance Tuning, Stale Statistics, Unusable Indexes.
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A “golden rule” when working with Automatic Indexing is that things don’t work properly if there are stale statistics on the dependant objects. Stale statistics can of course be problematic but they can be particularly troublesome when dealing with Automatic Indexing.

In the Oracle Autonomous Database environments, this issue is addressed somewhat by the new High Frequency Statistics Collection capability, which helps to automatically collect stale statistics on a regular basis. However, in on-prem Exadata environments where this can more easily be turned off or collected less frequently, it’s a potential issue worth consideration.

I’ll start with a simple little table, with a CODE column that has lots of distinct values:

SQL> create table bowie_stale (id number constraint bowie_stale_pk primary key, code number, name varchar2(42));

Table created.

SQL> insert into bowie_stale select rownum, mod(rownum, 1000000)+1, 'David Bowie' from dual connect by level <= 10000000;

10000000 rows created.

SQL> commit;

Commit complete.

Importantly, I don’t collect statistics on this newly populated table…

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

TABLE_NAME        NUM_ROWS     BLOCKS LAST_ANAL
--------------- ---------- ---------- ---------
BOWIE_STALE

SQL> select column_name, num_distinct, density, histogram, last_analyzed from user_tab_cols
where table_name='BOWIE_STALE';

COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       LAST_ANAL
-------------------- ------------ ---------- --------------- ---------
ID                                           NONE
CODE                                         NONE
NAME                                         NONE

If we now run the following query a number of times while there are no statistics on the table:

SQL> select * from bowie_stale where code=42;

10 rows selected.

Execution Plan

-----------------------------------------------------------------------------------------
| Id | Operation                | Name        | Rows | Bytes | Cost (%CPU)| Time        |
-----------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT         |             |  437 | 21413 |    553 (16)| 00:00:01    |
|* 1 | TABLE ACCESS STORAGE FULL| BOWIE_STALE |  437 | 21413 |    553 (16)| 00:00:01    |
-----------------------------------------------------------------------------------------

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

1 - storage("CODE"=42)
    filter("CODE"=42)

Note
-----
- dynamic statistics used: dynamic sampling (level=2)
- automatic DOP: Computed Degree of Parallelism is 1

Statistics
----------------------------------------------------------
          6  recursive calls
          0  db block gets
      39026  consistent gets
          0  physical reads
          0  redo size
        867  bytes sent via SQL*Net to client
        588  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         10  rows processed

 

The CBO has no choice but to use a FTS as I don’t yet have an index on the CODE column.

If I now wait for the next Automatic Indexing task to kick in AND if there are still NO statistics on the table:

 

SQL> select dbms_auto_index.report_last_activity('text', 'ALL', 'ALL' ) report from dual;

REPORT

--------------------------------------------------------------------------------
GENERAL INFORMATION
-------------------------------------------------------------------------------
Activity start               : 05-JUL-2020 06:36:31
Activity end                 : 05-JUL-2020 06:37:07
Executions completed         : 1
Executions interrupted       : 0
Executions with fatal error  : 0
-------------------------------------------------------------------------------

SUMMARY (AUTO INDEXES)
-------------------------------------------------------------------------------
Index candidates            : 1
Indexes created             : 0
Space used                  : 0 B
Indexes dropped             : 0
SQL statements verified     : 0
SQL statements improved     : 0
SQL plan baselines created  : 0
Overall improvement factor  : 0x
-------------------------------------------------------------------------------

SUMMARY (MANUAL INDEXES)
-------------------------------------------------------------------------------
Unused indexes    : 0
Space used        : 0 B
Unusable indexes  : 0

 

You can see that there was the one index candidate BUT no Automatic Index appears to have been created.

Assuming there are still no statistics:

 

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

TABLE_NAME                       NUM_ROWS    BLOCKS  LAST_ANAL
------------------------------ ---------- ---------- ---------
BOWIE_STALE

SQL> select column_name, num_distinct, density, histogram, last_analyzed from user_tab_cols
where table_name='BOWIE_STALE2';

COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       LAST_ANAL
-------------------- ------------ ---------- --------------- ---------
ID                                           NONE
CODE                                         NONE
NAME                                         NONE

 

If we look now at what indexes exist on the table:

SQL> select index_name, auto, constraint_index, visibility, compression, status, num_rows, leaf_blocks, clustering_factor
from user_indexes where table_name='BOWIE_STALE';

INDEX_NAME                     AUT CON VISIBILIT COMPRESSION   STATUS     NUM_ROWS LEAF_BLOCKS CLUSTERING_FACTOR
------------------------------ --- --- --------- ------------- -------- ---------- ----------- -----------------
BOWIE_STALE_PK                 NO  YES VISIBLE   DISABLED      VALID
SYS_AI_300kk2unp8tr0           YES NO  INVISIBLE DISABLED      UNUSABLE          0           0                 0

SQL> select index_name, column_name, column_position from user_ind_columns
where table_name='BOWIE_STALE2' order by index_name, column_position;

INDEX_NAME                     COLUMN_NAME          COLUMN_POSITION
------------------------------ -------------------- ---------------
BOWIE_STALE_PK                 ID                                 1
SYS_AI_300kk2unp8tr0           CODE                               1

 

We notice there is now an Automatic Index BUT it remains in an UNUSABLE/INVISIBLE state. This means the index can’t be used by the CBO.

So if we now re-run the SQL query again:

 

SQL> select * from bowie_stale where code=42;

10 rows selected.

Execution Plan

-----------------------------------------------------------------------------------------
| Id | Operation                | Name        | Rows | Bytes | Cost (%CPU)| Time        |
-----------------------------------------------------------------------------------------
|  0 | SELECT STATEMENT         |             |  437 | 21413 |    553 (16)| 00:00:01    |
|* 1 | TABLE ACCESS STORAGE FULL| BOWIE_STALE |  437 | 21413 |    553 (16)| 00:00:01    |
-----------------------------------------------------------------------------------------

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

1 - storage("CODE"=42)
    filter("CODE"=42)

Note
-----
- dynamic statistics used: dynamic sampling (level=2)
- automatic DOP: Computed Degree of Parallelism is 1

Statistics
----------------------------------------------------------
          6  recursive calls
          0  db block gets
      39026  consistent gets
          0  physical reads
          0  redo size
        867  bytes sent via SQL*Net to client
        588  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         10  rows processed

 

The CBO has no choice still but to use the FTS.

In Part II, we’ll see that once we get into this scenario, it can be a tad problematic to get ourselves out of it and get the Automatic Index created as we would like…

Oracle 19c Automatic Indexing: Data Skew Part II (Everything’s Alright) September 14, 2020

Posted by Richard Foote in 19c, 19c New Features, Automatic Indexing, Automatic Table Statistics, Autonomous Transaction Processing, Data Skew, Exadata, High Frequency Statistics Collection, Histograms, Oracle, Oracle Cost Based Optimizer, Oracle General, Oracle Indexes, Oracle Statistics, Performance Tuning.
4 comments

In my previous post, I discussed an example with data skew, in which the Automatic Indexing process created a new index, but somehow the CBO when using the index estimated the correct cardinality estimate even though no histograms were explicitly calculated.

In this post I’ll answer HOW this achieved by the CBO.

Get some idea on the answer by now looking at the column details:

SQL> select column_name, num_buckets, histogram from user_tab_cols
where table_name='BOWIE_SKEW';

COLUMN_NAME     NUM_BUCKETS HISTOGRAM
--------------- ----------- ---------------
ID                        1 NONE
CODE                     10 FREQUENCY
NAME                      1 NONE

We can see that there is now indeed an histogram on the column. When and how were these histograms collected?

The answer lies with a new Oracle Database 19c feature called “High-Frequency Automatic Statistics Collection“, which is available on Exadata environments. As I’m running all these demos on the Oracle Autonomous Transaction Processing Cloud environment which runs on an Exadata platform, this feature is enabled by default.

To highlight the capabilities of this features more fully, I’m going to setup a slightly different demo with three tables:

SQL> create table bowie1 (id number, code number, name varchar2(42));  <= Stale with no stats

Table created.

SQL> insert into bowie1 select rownum, mod(rownum, 100)+1, 'David Bowie' from dual connect by level <= 100000;

100000 rows created.

SQL> commit;

Commit complete.

 

Table BOWIE1 has no statistics collected on it.

 

SQL> create table bowie2 (id number, code number, name varchar2(42));

Table created.

SQL> insert into bowie2 select rownum, mod(rownum, 100)+1, 'David Bowie' from dual connect by level <= 100000;

100000 rows created.

SQL> commit;

Commit complete.

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

PL/SQL procedure successfully completed.

SQL> insert into bowie2 select rownum+100000, mod(rownum, 100)+1, 'Ziggy Stardust' from dual connect by level <= 50000;

50000 rows created.

SQL> commit;

Commit complete.

 

BOWIE2 table has new rows added after statistics have been collected and so has “stale” outdated stats.

 

SQL> create table bowie3 (id number, code number, name varchar2(42));

Table created.

SQL> insert into bowie3 select rownum, 10, 'DAVID BOWIE' from dual connect by level <=1000000;

1000000 rows created.

SQL> update bowie3 set code = 9 where mod(id,3) = 0;

333333 rows updated.

SQL> update bowie3 set code = 1 where mod(id,2) = 0 and id between 1 and 20000;

10000 rows updated.

SQL> update bowie3 set code = 2 where mod(id,2) = 0 and id between 30001 and 40000;

5000 rows updated.

SQL> update bowie3 set code = 3 where mod(id,100) = 0 and id between 300001 and 400000;

1000 rows updated.

SQL> update bowie3 set code = 4 where mod(id,100) = 0 and id between 400001 and 500000;

1000 rows updated.

SQL> update bowie3 set code = 5 where mod(id,100) = 0 and id between 600001 and 700000;

1000 rows updated.

SQL> update bowie3 set code = 6 where mod(id,1000) = 0 and id between 700001 and 800000;

100 rows updated.

SQL> update bowie3 set code = 7 where mod(id,1000) = 0 and id between 800001 and 900000;

100 rows updated.

SQL> update bowie3 set code = 8 where mod(id,1000) = 0 and id between 900001 and 1000000;

100 rows updated.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'bowie3', estimate_percent=>100, method_opt=>'FOR ALL COLUMNS SIZE 1');

PL/SQL procedure successfully completed.

SQL> select code, count(*) from bowie3 group by code order by code;

      CODE   COUNT(*)
---------- ----------
         1      10000
         2       5000
         3       1000
         4       1000
         5       1000
         6        100
         7        100
         8        100
         9     327235
        10     654465

 

The BOWIE3 table is as my previous example, with data skew but with NO histograms collected. I’m now going to run a query on BOWIE3 where the CBO gets the cardinality estimate hopelessly wrong because of the missing histogram on the CODE column:

SQL> select * from bowie3 where code=7;

100 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2517725203

----------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |   100K|  1953K|   974   (2)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| BOWIE3 |   100K|  1953K|   974   (2)| 00:00:01 |
----------------------------------------------------------------------------

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

1 - filter("CODE"=7)

 

If we look at the current statistics on these tables:

 

SQL> select table_name, num_rows, stale_stats, notes from user_tab_statistics
where table_name in ('BOWIE1', 'BOWIE2', 'BOWIE3');

TABLE_NAME        NUM_ROWS STALE_S NOTES
--------------- ---------- ------- ------------------------------
BOWIE1
BOWIE2              100000 YES
BOWIE3             1000000 NO
BOWIE2              150000         STATS_ON_CONVENTIONAL_DML

 

We can see that BOWIE1 has indeed no statistics.

BOWIE2 is marked as having state statistics, although thanks to another Oracle Database 19c feature called “Real-Time Statistics Collection“, does have some additional statistics captured (such as NUM_ROWS) when the additional rows were inserted. I’ll discuss this feature more fully in a later blog article.

BOWIE3 is considered fine in that it does have statistics which are NOT stale, BUT…

 

SQL> select column_name, num_buckets, histogram from user_tab_col_statistics
where table_name='BOWIE3';

COLUMN_NAME     NUM_BUCKETS HISTOGRAM
--------------- ----------- ---------------
ID                        1 NONE
CODE                      1 NONE
NAME                      1 NONE

We don’t currently have any histograms even though a simple single table query was previously run based on a CODE predicate which had hopelessly inaccurate cardinality estimates.

If we wait approximately 15 minutes (default) for the High-Frequency Automatic Statistics Collection process to run and look at these column statistics again:

SQL> select table_name, num_rows, stale_stats from user_tab_statistics
where table_name in ('BOWIE1', 'BOWIE2', 'BOWIE3');

TABLE_NAME        NUM_ROWS STALE_S
--------------- ---------- -------
BOWIE1              100000 NO
BOWIE2              150000 NO
BOWIE3             1000000 NO

SQL> select column_name, num_buckets, histogram from user_tab_col_statistics where table_name='BOWIE3';

COLUMN_NAME     NUM_BUCKETS HISTOGRAM
--------------- ----------- ---------------
ID                        1 NONE
CODE                     10 FREQUENCY
NAME                      1 NONE

 

We now notice that:

BOWIE1 now has statistics captured, as the High-Frequency Automatic Statistics Collection process looks for tables with missing statistics.

BOWIE2 now has fully up to date statistics, as the High-Frequency Automatic Statistics Collection process looks for tables with stale statistics.

BOWIE3 now has histograms on the CODE columns, as the High-Frequency Automatic Statistics Collection process looks out for missing histograms if queries have been subsequently run with poor cardinality estimates.

Having more accurate, appropriate and up to date statistics all supports the CBO in making much better decisions in relation to the use of any newly created Automatic Indexes.

 

You can configure High-Frequency Automatic Statistics Collection in the following manner:

SQL> EXEC DBMS_STATS.SET_GLOBAL_PREFS('AUTO_TASK_STATUS','ON');

PL/SQL procedure successfully completed.

This turns the feature ON/OFF. It’s OFF by default on standard Exadata environments but ON by default in Autonomous Database environment.

 

SQL> EXEC DBMS_STATS.SET_GLOBAL_PREFS('AUTO_TASK_MAX_RUN_TIME','900');

PL/SQL procedure successfully completed.

This configures how long to allow the process to run (default is 3600 seconds/60 minutes).

 

SQL> EXEC DBMS_STATS.SET_GLOBAL_PREFS('AUTO_TASK_INTERVAL','900');

PL/SQL procedure successfully completed.

This configures the interval between the process running (default is every 900 seconds/15 minutes).

 

In my next post, I’ll look at a slightly more complex data skew example with Automatic Indexing, where both selective and unselective SQL predicates are invoked…