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Estimate Index Size With Explain Plan (I Can’t Explain) April 24, 2014

Posted by Richard Foote in Estimate Index Size, Explain Plan For Index, Oracle Indexes.
9 comments

I discussed recently an updated MOS note that details the needs vs. the implications of rebuilding indexes.

Following is a neat little trick if you want to very quickly and cheaply estimate the size of an index if it were to be rebuilt or a new index before you actually create the thing. I meant to blog about this sometime ago but was re- reminded of it when I recently came across this entry in Connor McDonald’s excellent blog.

I’ll start by creating a table with a bunch of rows:

SQL> create table ziggy as select o.* from dba_objects o, dba_users;

Table created.

SQL> exec dbms_stats.gather_table_stats(ownname=>user, tabname=>'ZIGGY');

PL/SQL procedure successfully completed.

SQL> select count(*) from ziggy;

  COUNT(*)
----------
   3939187

I’m thinking of creating an index on the OBJECT_NAME column, but I’m unsure if I’ll have enough free space in my tablespace. So let’s quickly get an estimate of the index size by simply generating the explain plan of the CREATE INDEX statement:

SQL> explain plan for create index ziggy_object_name_i on ziggy(object_name);

Explained.

Elapsed: 00:00:00.09

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1219136602

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

| Id  | Operation              | Name                | Rows  | Bytes | Cost (%CPU)| Time     |

----------------------------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|   0 | CREATE INDEX STATEMENT |                     |  3939K|    93M| 22032   (3)| 00:00:01 |
|   1 |  INDEX BUILD NON UNIQUE| ZIGGY_OBJECT_NAME_I |       |       |            |          |
|   2 |   SORT CREATE INDEX    |                     |  3939K|    93M|            |          |
|   3 |    TABLE ACCESS FULL   | ZIGGY               |  3939K|    93M| 17199   (4)| 00:00:01 |

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------
Note
-----
   - estimated index size: 159M bytes

 

Notice the little note below the execution plan. Oracle has estimated an index size of approximately 159M bytes and it’s only taken it 0.09 seconds to do so. A trace of the session highlights how Oracle simply uses the table statistics in its determination of the estimated index size.

Well, that’s OK I have sufficient space for an index of that size.  Let’s create the physical index and check out its actual size:

SQL> create index ziggy_object_name_i on ziggy(object_name);

Index created.

SQL> select bytes from dba_segments where segment_name='ZIGGY_OBJECT_NAME_I';

     BYTES
----------
 163577856

SQL> analyze index ziggy_object_name_i validate structure;

Index analyzed.

SQL> select btree_space from index_stats;

BTREE_SPACE
-----------
  157875040

 

Not bad at all, the estimate and actual index sizes are pretty well spot on.

There are some limitations however. Let’s pick another column, SUBOBJECT_NAME, which has a large number of NULL values:

SQL> select count(*) from ziggy where subobject_name is not null;

  COUNT(*)
----------
     33669

SQL> explain plan for create index ziggy_subobject_name_i on ziggy(subobject_name);

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

Plan hash value: 4065057084

-------------------------------------------------------------------------------------------------
| Id  | Operation              | Name                   | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|   0 | CREATE INDEX STATEMENT |                        |  3939K|  7693K| 20132  (4)| 00:00:01 |
|   1 |  INDEX BUILD NON UNIQUE| ZIGGY_SUBOBJECT_NAME_I |       |       |     |          |
|   2 |   SORT CREATE INDEX    |                        |  3939K|  7693K|     |          |
|   3 |    TABLE ACCESS FULL   | ZIGGY                  |  3939K|  7693K| 17238  (4)| 00:00:01 |

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

-------------------------------------------------------------------------------------------------
Note
-----
   - estimated index size: 100M bytes

The SUBOBJECT_NAME column only has a relatively few (33,669) values that are not null, but the explain plan is still estimating the index to have the full 3.9 million rows (remembering that fully null indexed values are not indexed in a B-Tree index). The estimated index size of 100M is therefore not going to be particularly accurate.

SQL> create index ziggy_subobject_name_i on ziggy(subobject_name);

Index created.

SQL> select bytes from dba_segments where segment_name='ZIGGY_SUBOBJECT_NAME_I';

      
     BYTES
----------
   1048576

SQL> analyze index ziggy_subobject_name_i validate structure;

Index analyzed.

SQL> select btree_space from index_stats;

BTREE_SPACE
-----------
     928032

So in this example, the estimated index size is indeed way off. This method doesn’t seem to cater for null index values and assumes the index to be fully populated.

However, if we simply take the known ratio of  not null values (in this example, 33669 not null rows /3939187 total rows =0.00855) and then apply it to the calculated estimate (100M x .00855 = 0.855M), where are now back into accurate ballpark territory again.

Of course, such estimates are based on the accuracy of the table statistics. If we have stale statistics, we’ll have stale index size estimates.

Let’s insert more rows and double the size of the table and associated index: 

SQL> insert into ziggy select * from ziggy;

3939187 rows created.

SQL> commit;

Commit complete.

If we re-run the index creation explain plan:

SQL> explain plan for create index ziggy_object_name_i on ziggy(object_name);

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

Plan hash value: 746589531

----------------------------------------------------------------------------------------------
| Id  | Operation              | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|   0 | CREATE INDEX STATEMENT |                     |  3939K|    93M| 22032   (3)| 00:00:01 |
|   1 |  INDEX BUILD NON UNIQUE| ZIGGY_OBJECT_NAME_I |       |       |            |          |
|   2 |   SORT CREATE INDEX    |                     |  3939K|    93M|            |          |
|   3 |    INDEX FAST FULL SCAN| ZIGGY_OBJECT_NAME_I |       |       |            |          |

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------
Note
-----
   - estimated index size: 159M bytes

We get the same estimate as before. We need to update the table statistics in order to get an updated and more accurate index size estimate:

SQL> exec dbms_stats.gather_table_stats(ownname=>user, tabname=>'ZIGGY');

PL/SQL procedure successfully completed.

SQL> explain plan for create index ziggy_object_name_i on ziggy(object_name);

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

Plan hash value: 746589531

----------------------------------------------------------------------------------------------
| Id  | Operation              | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

|   0 | CREATE INDEX STATEMENT |                     |  7878K|   187M| 45811   (3)| 00:00:02 |
|   1 |  INDEX BUILD NON UNIQUE| ZIGGY_OBJECT_NAME_I |       |       |            |          |
|   2 |   SORT CREATE INDEX    |                     |  7878K|   187M|            |          |
|   3 |    INDEX FAST FULL SCAN| ZIGGY_OBJECT_NAME_I |       |       |            |          |


PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------
Note
-----
   - estimated index size: 318M bytes

Both the estimated index entries and  index size are now much more accurate.

The number of expected index entries is therefore a useful guide as to the potential accuracy of the size estimate.

So the next time you’re wondering whether an index is significantly larger than it should or whether you have sufficient space for a new index, this is a useful, simple technique to get a quick estimate.

Presenting at ODTUG Kscope14 Conference in Seattle June 22-26 2014 April 23, 2014

Posted by Richard Foote in ODTUG Kscope14.
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Kscope14

 

Just a short note to say I’ll be presenting at the Oracle Development Tools User Group (ODTUG) Kaleidoscope 14 Conference this year in beautiful Seattle, Washington on June 22-26 2014. I had a fantastic time when I attended this conference a few years ago when it was held in Monterey so I’m really looking forward to it. It has another great lineup of speakers this year including Tom Kyte, Jonathan Lewis, Cary Millsap, Tim Gorman, Alex Gorbachev, Kyle HaileyKellyn Pot’Vin, Yury Velikanov and Bryn Llewellyn to name but a very few so it should be another excellent event.

I’ll be presenting a couple of papers and perhaps be on the odd panel or two.

  • New Indexing Features Introduced In Oracle Database 12c (Monday June 23, Session 3 2:30 pm – 3:30 pm)
  • Indexing In Exadata (Tuesday June 24, Session 8 2 pm – 3 pm)

Hope to see some of you there although sadly it looks like Stanley The ACE Vest won’t be able to make it this time :)

Indexing Foreign Key Constraints With Invisible Indexes (Invisible People) April 22, 2014

Posted by Richard Foote in 12c, Block Dumps, Foreign Keys, Invisible Indexes, Oracle Indexes.
1 comment so far

In my previous post I discussed when deleting rows from parent tables, how Bitmap Indexes based on the FK constraint can prevent the expensive Full Tables Scans (FTS) on the child tables but not the associated exclusive table locks.

Last year, I discussed how it was possible in Oracle Database 12c to have multiple indexes on the same column list.

Quite some time ago, I discussed how so-called Invisible Indexes can indeed still be visible in various scenarios, including when policing FK constraints.

Well, lets put all these three topics together :)

First, let use the same basic setup as the last post:

SQL> create table bowie_dad (id number, dad_name varchar2(30)); 

Table created.

SQL> insert into bowie_dad values (1, 'DAVID BOWIE'); 

1 row created.

SQL> insert into bowie_dad values (2, 'ZIGGY STARDUST'); 

1 row created.

SQL> insert into bowie_dad values (3, 'MAJOR TOM');

1 row created.

SQL> insert into bowie_dad values (4, 'THIN WHITE DUKE');

1 row created.

SQL> commit;

Commit complete.

SQL> create table bowie_kid (id number, kid_name varchar2(30), dad_id number);

Table created.

SQL> insert into bowie_kid select rownum, 'ALADDIN SANE', mod(rownum,3)+2 from dual connect by level <=1000000; 

1000000 rows created.

SQL> commit;

Commit complete.

SQL> alter table bowie_dad add primary key(id);

Table altered.

SQL> alter table bowie_kid add constraint bowie_kid_fk foreign key(dad_id) references bowie_dad(id);

Table altered.

 

We’re now going to create two indexes concurrently on the FK constraint on the DAD_ID column, a Bitmap Index and an invisible B-Tree Index as is now possible since Oracle Database 12c:

SQL> create bitmap index bowie_kid_fk_i on bowie_kid(dad_id); 

Index created.

SQL> create index bowie_kid_fk2_i on bowie_kid(dad_id) invisible;

Index created.

Oracle Database 12c allows us to now create multiple indexes on the same column list, providing only one index is visible at a time.

Let’s look at a partial block dump of the first leaf block of each index. First the Bitmap Index:

Block header dump:  0x0180805c
 Object id on Block? Y
 seg/obj: 0x16f45  csc: 0x00.36bc54  itc: 2  flg: E  typ: 2 – INDEX
     brn: 0  bdba: 0x1808058 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0000.000.00000000  0x00000000.0000.00  —-    0  fsc 0x0000.00000000
0x02   0xffff.000.00000000  0x00000000.0000.00  C—    0  scn 0x0000.0036bc54
Leaf block dump
===============
header address 32801380=0x1f48264
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x80: opcode=0: iot flags=— is converted=Y
kdxconco 4
kdxcosdc 0
kdxconro 2
kdxcofbo 40=0x28
kdxcofeo 959=0x3bf
kdxcoavs 919
kdxlespl 0
kdxlende 0
kdxlenxt 25198685=0x180805d
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[4499] flag: ——-, lock: 0, len=3537
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 7f d3 00 00
col 2; len 6; (6):  01 80 80 2c 00 3f
col 3; len 3516; (3516):
 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92
 24 49 cf 92 24 49 92 24 49 92 24 cc 49 92 24 49 02 ff 32 24 49 92 24 49 92
 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24
 49 92 24 49 cb 92 24 49 92 ff 33 24 49 92 24 49 92 24 49 cf 92 24 49 92 24
 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cc 92 24 49
 92 24 ff 32 24 49 92 24 49 92 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24
 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cb 92 24 49 92 ff 33 49 92 24 49

 

Note the indexed value is c1 03, denoting the lowest DAD_ID=2 currently in the table.

Now the  partial block dump of the invisible B-Tree Index:

 

Block header dump:  0x0181b724
 Object id on Block? Y
 seg/obj: 0x16f46  csc: 0x00.36bc78  itc: 2  flg: E  typ: 2 – INDEX
     brn: 0  bdba: 0x181b720 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0000.000.00000000  0x00000000.0000.00  —-    0  fsc 0x0000.00000000
0x02   0xffff.000.00000000  0x00000000.0000.00  C—    0  scn 0x0000.0036bc78
Leaf block dump
===============
header address 32801380=0x1f48264
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x80: opcode=0: iot flags=— is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 513
kdxcofbo 1062=0x426
kdxcofeo 1880=0x758
kdxcoavs 818
kdxlespl 0
kdxlende 0
kdxlenxt 25278245=0x181b725
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[8024] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 7f d3 00 01
row#1[8012] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 7f d3 00 04
row#2[8000] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 7f d3 00 07

Again as expected the first index entry is C1 03.

With only a visible Bitmap Index in place, does that mean we’ll have table locking issues if we delete a parent row with current transactions in place ? Let’s check it out.

In one session, we have a current transaction on the child table:

SQL> insert into bowie_kid values (1000001, 'LOW', 4); 

1 row created.

 

In another session, we attempt to delete a parent row (with an ID = 1 which doesn’t currently exist with the child table):

SQL> delete bowie_dad where id = 1; 

1 row deleted.

We note the DML was successful and didn’t hang. This means the B-Tree index is clearly being used to police this constraint, even though it’s currently invisible.

In a third session, we now attempt to insert a child row using a FK value that’s in the process of being deleted:

SQL> insert into bowie_kid values (1000003, 'HEROES', 1);

 

As expected, it hangs as it’s currently effectively waiting on the row level lock made possible by the index entry in the B-Tree index as invisible indexes are still maintained behind the scenes. If we look at a fresh block dump of both indexes, beginning with the Bitmap Index:

Block header dump:  0x0180805c
 Object id on Block? Y
 seg/obj: 0x16f45  csc: 0x00.36bc54  itc: 2  flg: E  typ: 2 – INDEX
     brn: 0  bdba: 0x1808058 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0000.000.00000000  0x00000000.0000.00  —-    0  fsc 0x0000.00000000
0x02   0xffff.000.00000000  0x00000000.0000.00  C—    0  scn 0x0000.0036bc54
Leaf block dump
===============
header address 402948708=0x18048264
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x80: opcode=0: iot flags=— is converted=Y
kdxconco 4
kdxcosdc 0
kdxconro 2
kdxcofbo 40=0x28
kdxcofeo 959=0x3bf
kdxcoavs 919
kdxlespl 0
kdxlende 0
kdxlenxt 25198685=0x180805d
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[4499] flag: ——-, lock: 0, len=3537
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 7f d3 00 00
col 2; len 6; (6):  01 80 80 2c 00 3f
col 3; len 3516; (3516):
 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92
 24 49 cf 92 24 49 92 24 49 92 24 cc 49 92 24 49 02 ff 32 24 49 92 24 49 92
 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24
 49 92 24 49 cb 92 24 49 92 ff 33 24 49 92 24 49 92 24 49 cf 92 24 49 92 24
 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cc 92 24 49
 92 24 ff 32 24 49 92 24 49 92 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24
 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cb 92 24 49 92 ff 33 49 92 24 49

 

We note the Bitmap Index has not been updated. It still lists the C1 03 value as the minimum indexed value.

However, if we look at the invisible B-Tree index:

Block header dump:  0x0181b724
 Object id on Block? Y
 seg/obj: 0x16f46  csc: 0x00.36bc78  itc: 2  flg: E  typ: 2 – INDEX
     brn: 0  bdba: 0x181b720 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0000.000.00000000  0x00000000.0000.00  —-    0  fsc 0x0000.00000000
0x02   0x0008.015.00000b86  0x014316ab.01c5.42  —-    1  fsc 0x0000.00000000
Leaf block dump
===============
header address 402948708=0x18048264
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x80: opcode=0: iot flags=— is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 514
kdxcofbo 1064=0x428
kdxcofeo 1868=0x74c
kdxcoavs 804
kdxlespl 0
kdxlende 0
kdxlenxt 25278245=0x181b725
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[1868] flag: ——-, lock: 2, len=12
col 0; len 2; (2):  c1 02
col 1; len 6; (6):  01 81 b6 f3 00 00
row#1[8024] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 7f d3 00 01
row#2[8012] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 7f d3 00 04
row#3[8000] flag: ——-, lock: 0, len=12

It has been updated and lists a new index entry C1 02 as the minimum value now in the index.

So the B-Tree index can be used to successfully police the FK index and prevent the possible table level locking issues associated with deleting parent rows, even though it’s invisible and there is an equivalent visible Bitmap index in place. Invisible indexes are simply not considered as viable execution paths by the Cost Based Optimizer, but may still be “visible” in a variety of scenarios such as quietly policing constraints behind the scenes.

Do I recommend creating two such indexes in Oracle Database 12c. Well, no as the costs of maintaining both indexes need to be considered. But I certainly do caution simply making indexes invisible and expecting the database to behave in exactly the same manner if the index were to be subsequently dropped.

Because rolling back all the above and then dropping the invisible index:

SQL> drop index bowie_kid_fk2_i;

Index dropped.

SQL> insert into bowie_kid values (1000001, 'LOW', 4);

1 row created.

 

Means in another session the parent delete operation will now hang without the B-Tree index being in place:

SQL> delete bowie_dad where id = 1;

 

 

 

 

 

 

Indexing Foreign Key Constraints With Bitmap Indexes (Locked Out) April 17, 2014

Posted by Richard Foote in Bitmap Indexes, Block Dumps, Foreign Keys, Index Internals, Oracle Indexes.
5 comments

Franck Pachot made a very valid comment in my previous entry on Indexing Foreign Keys (FK) that the use of a Bitmap Index on the FK columns does not avoid the table locks associated with deleting rows from the parent table. Thought I might discuss why this is the case and why only a B-Tree index does the trick.

Let’s first setup some very simple Parent-Child tables:

SQL> create table bowie_dad (id number, dad_name varchar2(30));

Table created.

SQL> insert into bowie_dad values (1, 'DAVID BOWIE');

1 row created.

SQL> insert into bowie_dad values (2, 'ZIGGY STARDUST');

1 row created.

SQL> insert into bowie_dad values (3, 'MAJOR TOM');

1 row created.

SQL> insert into bowie_dad values (4, 'THIN WHITE DUKE');

1 row created.

SQL> commit;

Commit complete.

SQL> create table bowie_kid (id number, kid_name varchar2(30), dad_id number);

Table created.

SQL> insert into bowie_kid select rownum, 'ALADDIN SANE', mod(rownum,3)+2 from dual connect by level >=1000000;

1000000 rows created.

SQL> commit;

Commit complete.

SQL> alter table bowie_dad add primary key(id);

Table altered.

SQL> alter table bowie_kid add constraint bowie_kid_fk foreign key(dad_id) references bowie_dad(id);

Table altered.

OK, so we have a small parent table (BOWIE_DAD) and a much larger child table (BOWIE_KID) with all the necessary constraints in place. Note we don’t actually have a child row with a  FK DAD_ID = 1. So we can potentially delete this row from the BOWIE_DAD table (where ID = 1).

Let’s begin by creating a B-Tree  index on the FK column (DAD_ID) and have a look a partial block dump of the first leaf block in the index:

SQL> create index bowie_kid_fk_i on bowie_kid(dad_id);

Index created.

 

Block header dump:  0x01806efc
 Object id on Block? Y
 seg/obj: 0x16f0b  csc: 0x00.35f861  itc: 2  flg: E  typ: 2 – INDEX
     brn: 0  bdba: 0x1806ef8 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0000.000.00000000  0x00000000.0000.00  —-    0  fsc 0x0000.00000000
0x02   0xffff.000.00000000  0x00000000.0000.00  C—    0  scn 0x0000.0035f861
Leaf block dump
===============
header address 360809060=0x15818264
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x80: opcode=0: iot flags=— is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 513
kdxcofbo 1062=0x426
kdxcofeo 1880=0x758
kdxcoavs 818
kdxlespl 0
kdxlende 0
kdxlenxt 25194237=0x1806efd
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[8024] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 00
row#1[8012] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 03
row#2[8000] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 06
…..

 

We’ll compare future block dumps with this one but for now just note that the first index entry has a value of (hex) C1 03, which corresponds to the minimum value for DAD_ID = 2 we currently have in this table/index.

If we insert a new child record in one session (but not yet commit);

SQL> insert into bowie_kid values (1000001, 'LOW', 4);

1 row created.

In a second session, we can delete (but not yet commit) the unwanted parent row without any locking implications thanks to this index on the FK column:

SQL> delete bowie_dad where id = 1;

1 row deleted.

In a third session, we can insert another child record again with no locking implications, providing we don’t attempt to use the parent value the second session is in the process of deleting:

SQL> insert into bowie_kid values (1000002, 'LOW', 3);

1 row created.

But if we do try to insert a new child row with a FK value for which the parent is in the process of being deleted:

SQL> insert into bowie_kid values (1000003, 'HEROES', 1);

The statement hangs and it will do so until the transaction deleting the parent record commits (in which case it will receive an ORA-02291 integrity constraint error) or the transaction rolls back (in which case the insert will succeed).

If we take a fresh dump of the first leaf block (which must contain the associated index entry as it’s the minimum value now in the table):

 Block header dump:  0x01806efc
 Object id on Block? Y
 seg/obj: 0x16f0b  csc: 0x00.35f861  itc: 2  flg: E  typ: 2 – INDEX
     brn: 0  bdba: 0x1806ef8 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0000.000.00000000  0x00000000.0000.00  —-    0  fsc 0x0000.00000000
0x02   0x0008.004.00000b8a  0x01431602.01c5.14  —-    1  fsc 0x0000.00000000
Leaf block dump
===============
header address 225280612=0xd6d8264
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x80: opcode=0: iot flags=— is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 514
kdxcofbo 1064=0x428
kdxcofeo 1868=0x74c
kdxcoavs 804
kdxlespl 0
kdxlende 0
kdxlenxt 25194237=0x1806efd
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[1868] flag: ——-, lock: 2, len=12
col 0; len 2; (2):  c1 02
col 1; len 6; (6):  01 80 7f 38 00 00
row#1[8024] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 00
row#2[8012] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 03

 

We notice we indeed do have a new index entry (highlighted above), with all the associated locking information in ITL slot 2 for the new row in which the session is locked. So the key point here is that the index is indeed updated and Oracle can proceed or not depending on what happens with the transaction on the parent table. The overhead of this new index entry is minimal and locking can be easily policed and restricted to just the index entries with this specific value (hex) C1 02 which corresponds to DAD_ID = 1.

If we do indeed proceed with the delete on the parent table:

SQL> commit;

Commit complete.

 

The session attempting to insert the now deleted parent FK value indeed fails:

 

SQL> insert into bowie_kid values (1000002, 'HEROES', 1);
insert into bowie_kid values (1000002, 'HEROES', 1)
*
ERROR at line 1:
ORA-02291: integrity constraint (BOWIE.BOWIE_KID_FK) violated - parent key not
found

 

And we notice with a fresh block dump that the index entry has been removed by the now unlocked session:

 

Block header dump:  0x01806efc
 Object id on Block? Y
 seg/obj: 0x16f0b  csc: 0x00.35f861  itc: 2  flg: E  typ: 2 – INDEX
     brn: 0  bdba: 0x1806ef8 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0000.000.00000000  0x00000000.0000.00  —-    0  fsc 0x0000.00000000
0x02   0xffff.000.00000000  0x00000000.0000.00  C—    0  scn 0x0000.0035f861
Leaf block dump
===============
header address 225280612=0xd6d8264
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x80: opcode=0: iot flags=— is converted=Y
kdxconco 2
kdxcosdc 0
kdxconro 513
kdxcofbo 1062=0x426
kdxcofeo 1880=0x758
kdxcoavs 818
kdxlespl 0
kdxlende 0
kdxlenxt 25194237=0x1806efd
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[8024] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 00
row#1[8012] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 03
row#2[8000] flag: ——-, lock: 0, len=12
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 06

Everything is back to the way it was previously.

 

OK, let’s now re-insert the parent row, drop the FK index and replace it with a Bitmap Index instead:

 

SQL> insert into bowie_dad values (1, 'DAVID BOWIE');

1 row created.

SQL> commit;

Commit complete.

SQL> drop index bowie_kid_fk_i;

Index dropped.

SQL> create bitmap index bowie_kid_fk_i on bowie_kid(dad_id);

Index created.

 

If we take a look at a partial block dump of the first leaf block of this Bitmap Index:

 

Block header dump:  0x01806efc
 Object id on Block? Y
 seg/obj: 0x16f14  csc: 0x00.3602fc  itc: 2  flg: E  typ: 2 – INDEX
     brn: 0  bdba: 0x1806ef8 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0000.000.00000000  0x00000000.0000.00  —-    0  fsc 0x0000.00000000
0x02   0xffff.000.00000000  0x00000000.0000.00  C—    0  scn 0x0000.003602fc
Leaf block dump
===============
header address 360809060=0x15818264
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x80: opcode=0: iot flags=— is converted=Y
kdxconco 4
kdxcosdc 0
kdxconro 2
kdxcofbo 40=0x28
kdxcofeo 958=0x3be
kdxcoavs 918
kdxlespl 0
kdxlende 0
kdxlenxt 25194237=0x1806efd
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[4498] flag: ——-, lock: 0, len=3538
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 00
col 2; len 6; (6):  01 80 6e cc 00 3f
col 3; len 3517; (3517):
 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cf 92 24 49 92 24 49
 92 24 cf 49 92 24 49 92 24 49 92 cc 24 49 92 24 01 ff 32 92 24 49 92 24 49
 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cf 92 24 49 92
 24 49 92 24 cc 49 92 24 49 02 ff 32 24 49 92 24 49 92 24 49 cf 92 24 49 92
 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cb 92 24
 49 92 ff 33 24 49 92 24 49 92 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24
 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cc 92 24 49 92 24 ff 32 24 49 92
 24 49 92 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24
 49 92 24 49 92 24 49 cb 92 24 49 92 ff 33 92 24 49 92 24 49 92 24 cf 49 92
 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cf 92 24 49 92 24 49 92 24 cc
 49 92 24 49 02 ff 32 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cf
 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cc 24 49 92 24 01 ff 32
 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24
 49 cf 92 24 49 92 24 49 92 24 cc 49 92 24 49 02 ff 32 24 49 92 24 49 92 24
 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49
 92 24 49 cb 92 24 49 92 ff 33 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92
 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cc 24 49 92 24
 01 ff 32 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24
 49 92 24 49 cf 92 24 49 92 24 49 92 24 cc 49 92 24 49 02 ff 32 24 49 92 24

….

 

We notice the first key difference here in that these Bitmap Index entries are potentially HUGE, with just the 2 index entries in this block. The other thing to note is the combination of Bitmap indexes and DMLs can result in locking hell because if an index entry needs to be modified (resulting in a change in the compressed bitmap string), all rows between the rowid ranges specified within the Bitmap Index entry are effectively locked. So Bitmap Indexes introduce severe locking issues, regardless of the Parent/Child update issue highlighted above.

If we insert a child row in one session:

SQL> insert into bowie_kid values (1000001, 'LOW', 4);

1 row created.

And in another session insert another row with the same FK value:

SQL> insert into bowie_kid values (1000002, 'HEROES', 4);

The session hangs until the transaction in the first session completes because of the locking implications introduced with the Bitmap Index.

 

Therefore, with a Bitmap Index in place, the last of our worries will be locking issues associated with deleting a parent row. After rolling back the above, we attempt the following. In one session, we insert a child record:

SQL> insert into bowie_kid values (1000001, 'LOW', 4);

1 row created.

In a second session, we delete the unwanted parent row:

SQL> delete bowie_dad where id = 1;

and it hangs. The Bitmap Index is not effective in preventing this lock as it was with the B-Tree Index.

In a third session, we attempt to insert a child row with the soon to be deleted parent key:

SQL> insert into bowie_kid values (1000002, 'HEROES', 1);

and it hangs as well. So the Bitmap Index on the FK does not prevent the locking hell such parent deletes can introduce into our environments.

If we roll all this back and simply have one session delete a parent row:

SQL> delete bowie_dad where id = 1;

1 row deleted.

And in another session insert a child row with the FK about to be deleted, the insert hangs as expected with an exclusive transaction lock:

SQL> insert into bowie_kid values (1000001, 'BOWIE', 1);

 

However, if we look at a fresh partial block dump of the first Bitmap Index leaf block:

Block header dump:  0x01806efc
 Object id on Block? Y
 seg/obj: 0x16f14  csc: 0x00.3602fc  itc: 2  flg: E  typ: 2 – INDEX
     brn: 0  bdba: 0x1806ef8 ver: 0x01 opc: 0
     inc: 0  exflg: 0
 
 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0000.000.00000000  0x00000000.0000.00  —-    0  fsc 0x0000.00000000
0x02   0xffff.000.00000000  0x00000000.0000.00  C—    0  scn 0x0000.003602fc
Leaf block dump
===============
header address 225280612=0xd6d8264
kdxcolev 0
KDXCOLEV Flags = – – -
kdxcolok 0
kdxcoopc 0x80: opcode=0: iot flags=— is converted=Y
kdxconco 4
kdxcosdc 0
kdxconro 2
kdxcofbo 40=0x28
kdxcofeo 958=0x3be
kdxcoavs 918
kdxlespl 0
kdxlende 0
kdxlenxt 25194237=0x1806efd
kdxleprv 0=0x0
kdxledsz 0
kdxlebksz 8036
row#0[4498] flag: ——-, lock: 0, len=3538
col 0; len 2; (2):  c1 03
col 1; len 6; (6):  01 80 52 73 00 00
col 2; len 6; (6):  01 80 6e cc 00 3f
col 3; len 3517; (3517):
 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cf 92 24 49 92 24 49
 92 24 cf 49 92 24 49 92 24 49 92 cc 24 49 92 24 01 ff 32 92 24 49 92 24 49
 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cf 92 24 49 92
 24 49 92 24 cc 49 92 24 49 02 ff 32 24 49 92 24 49 92 24 49 cf 92 24 49 92
 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cb 92 24
 49 92 ff 33 24 49 92 24 49 92 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24
 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cc 92 24 49 92 24 ff 32 24 49 92
 24 49 92 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24
 49 92 24 49 92 24 49 cb 92 24 49 92 ff 33 92 24 49 92 24 49 92 24 cf 49 92
 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cf 92 24 49 92 24 49 92 24 cc
 49 92 24 49 02 ff 32 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24 49 cf
 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cc 24 49 92 24 01 ff 32
 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92 24
 49 cf 92 24 49 92 24 49 92 24 cc 49 92 24 49 02 ff 32 24 49 92 24 49 92 24
 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24 49
 92 24 49 cb 92 24 49 92 ff 33 49 92 24 49 92 24 49 92 cf 24 49 92 24 49 92
 24 49 cf 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cc 24 49 92 24
 01 ff 32 92 24 49 92 24 49 92 24 cf 49 92 24 49 92 24 49 92 cf 24 49 92 24
 49 92 24 49 cf 92 24 49 92 24 49 92 24 cc 49 92 24 49 02 ff 32 24 49 92 24

…..

 

Unlike the B-Tree index which was updated, the Bitmap index has remained unchanged. No attempt was made by Oracle at this stage to insert the index entry as such a new Bitmap Index entry would likely generate too much overheads and not appreciably reduce the locking implications of these DML statements with these Bitmap Indexes in place anyways. The actual index update is delayed until such as change is possible with the rollback of the parent deletion.

However, in a third session, an insert into the child table with a FK that’s not to be deleted is successful:

SQL> insert into bowie_kid values (1000002, 'BOWIE', 4);

1 row created.

Bitmap indexes are simply not designed with concurrency in mind and have efficiencies that make it easier for single sessions to load data in Data Warehouses environments where they are indeed suitable.

One advantage of the Bitmap index is that at least Oracle doesn’t have to perform a FTS on the (potentially huge) child table when checking for the existence of any associated child FK values. Oracle can quickly use the index to determine whether the parent delete can proceed or not. If we roll everything back and just attempt to delete a parent row:

SQL> delete bowie_dad where id = 1;

1 row deleted.

       
Execution Plan
----------------------------------------------------------
Plan hash value: 2571176721

-----------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | DELETE STATEMENT   |              |     1 |    13 |     0   (0)| 00:00:01 |
|   1 |  DELETE            | BOWIE_DAD    |       |       |            |          |
|*  2 |   INDEX UNIQUE SCAN| SYS_C0010356 |     1 |    13 |     0   (0)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access('ID'=1)

    
Statistics
----------------------------------------------------------
          7  recursive calls
          8  db block gets
          3  consistent gets
          0  physical reads
        676  redo size
        862  bytes sent via SQL*Net to client
        830  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          1  rows processed

 

We notice at just 3 consistent gets, the potentially expensive FTS on the child table has been avoided. Drop the Bitmap index and the FTS must be performed to ensure no current FK values would violate the constraint when the parent row is deleted:

SQL> drop index bowie_kid_fk_i;

Index dropped.

   
SQL> delete bowie_dad where id = 1;

1 row deleted.

    
Execution Plan
----------------------------------------------------------
Plan hash value: 2571176721

-----------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | DELETE STATEMENT   |              |     1 |    13 |     0   (0)| 00:00:01 |
|   1 |  DELETE            | BOWIE_DAD    |       |       |            |          |
|*  2 |   INDEX UNIQUE SCAN| SYS_C0010356 |     1 |    13 |     0   (0)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access('ID'=1)

    
Statistics
----------------------------------------------------------
          7  recursive calls
          8  db block gets
       3629  consistent gets
          0  physical reads
        676  redo size
        863  bytes sent via SQL*Net to client
        830  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          1  rows processed

 

We notice without the Bitmap Index in place, we are now performing many more (3629) consistent gets due to the necessary FTS.

So using a Bitmap Index to police a FK constraint doesn’t reduce the locking implications associated with deleting parent rows (with Bitmap indexes, we have locking hell regardless if there’s much DML) but it does at least reduce the overheads of checking the associated child table.

Indexing Foreign Keys (Helden) April 2, 2014

Posted by Richard Foote in Foreign Keys, Oracle Indexes.
6 comments

A recent question on an internal forum asked whether an index on a Foreign Key (FK) constraint designed to avoid locking issues associated with deletes on the parent tables needs to precisely match the columns in the FK. Could the columns in the index potentially be a different order or be appended with additional columns ?

The answer is basically the same as when using an index to police a Primary Key or Unique Key constraint. An index can be used providing the leading columns match those of the constraint (in any order). The index can indeed potentially have additional columns appended (or overloaded) to it.

Often the easiest way to find out these sorts of things is of course to just test it :) The point of this blog is not only to show candidate FK based indexes but also to highlight how easy it is to create simple test cases.

First, let’s create a couple of tables:

SQL> CREATE TABLE artists (id NUMBER,
                           code number,
                           artist_name VARCHAR2(30));

Table created.

SQL> CREATE TABLE albums (id NUMBER,
                          album_name VARCHAR2(30),
                          artist_id NUMBER ,
                          artist_code number,
                          format_id number);

Table created.

We populate the ARTISTS parent table with a few rows:

SQL> INSERT INTO artists VALUES (1, 1, 'DAVID BOWIE'); 

1 row created.

SQL> INSERT INTO artists VALUES (1, 2, 'ZIGGY STARDUST'); 

1 row created.

SQL> INSERT INTO artists VALUES (2, 1, 'MAJOR TOM');

1 row created.

SQL> INSERT INTO artists VALUES (2, 2, 'THIN WHITE DUKE');

1 row created.

We now populate the much larger ALBUMS child table with lots of rows:

SQL> insert into albums select rownum, 'BLAH', 1,  mod(rownum,2)+1, mod(rownum,100)
from dual connect by level <= 1000000;

1000000 rows created.

SQL> commit;

Commit complete.

Now the tables are populated, we can add the necessary constraints. A concatenated PK based on the ID and CODE columns on the ARTISTS table and an associated FK constraint on the ALBUMS table:

SQL> alter table artists add primary key (id, code); 

Table altered.

SQL> alter table albums add constraint artists_fk foreign key (artist_id, artist_code)
references artists(id, code);

Table altered.

OK, note at this point there is no index based on the FK constraint columns on the ALBUMS table. Let’s look at the number of consistent gets generated when we try to delete just a single row from the tiny ARTISTS table:

SQL> delete artists where id=2 and code = 1; 

1 row deleted.

Execution Plan
----------------------------------------------------------
Plan hash value: 898601404

-----------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | DELETE STATEMENT   |              |     1 |    26 |     0   (0)| 00:00:01 |
|   1 |  DELETE            | ARTISTS      |       |       |            |          |
|*  2 |   INDEX UNIQUE SCAN| SYS_C0010352 |     1 |    26 |     0   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

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

   2 - access("ID"=2 AND "CODE"=1)

Statistics
----------------------------------------------------------
          8  recursive calls
          7  db block gets
       3358  consistent gets
          0  physical reads
        640  redo size
        864  bytes sent via SQL*Net to client
        839  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL> rollback;

Rollback complete.

We notice our first issue. In order to delete just one row from the table via a Unique Index scan, we performed a massive 3358 consistent gets. Why? Because we can only successfully delete this row if there are no corresponding FKs based on this parent row. Without an associated index, the only way Oracle can perform this check on the large child table is to perform an expensive, slow, Full Table Scan (FTS).

Let’s rollback and this time start with an insert into the child, ALBUMS table (but not yet commit): 

SQL> insert into albums values (1000001, 'HEATHEN', 1, 1, 1); 

1 row created.

In a second session, let’s now attempt to delete the a parent row from the ARTISTS table:

SQL> delete artists where id = 2;

 

We notice, this session now hangs while it waits for all current transactions on the ALBUMS table complete.

In a third session, we attempt to insert another row into the child, ALBUMS table:

SQL> insert into albums values (1000002, 'THE NEXT DAY', 1,2,3);

 

And we notice it hangs as well, due to the previous locks on the table. With lots of other transactions trying to make changes to the ALBUMS table getting locked as well, we effectively have locking hell …

Why ? Because Oracle needs some way to ensure while it runs the FTS looking for any FKs associated with the deleted parent row, no-one else comes in and inserts or updates a row with this FK value. And as Oracle is performing a slow FTS, and values could potentially be inserted or updated in an area of the table already checked, the only way to effectively achieve this is to exclusively lock the table. And exclusive table locks are not really that great from a concurrency point of view …

So, introducing the index on the FK column(s). By having an index in place, we can effectively address both of the above issues. When searching for a corresponding FK value, the index will very quickly direct us to the leaf block that will either:

  • find a value of the parent key being deleted (in which case the delete of the parent row will fail with an ORA-02292 that a child record has been found) or
  • not find the value being deleted, in which case the delete on the parent row can be successful

Additionally, as it’s a very fast index scan being performed, there is no need to exclusively lock the table. Oracle in fact effectively “locks” the location within the index where the index value would reside if it existed or were to be subsequently inserted. Only an attempt to insert/update a row into the child table with the specific deleted FK value would be locked until the point when the parent delete is either committed (in which case the child insert will fail with an ORA-02291 parent key not found) or rolled back (in which case the child insert will be successful). The FK index can effectively detect when such a child insert has taken place because unlike the table where a such a new row could potentially be anywhere within the table, such an insert can only occur in a specific location within the index.

So if you do potentially delete a parent record (or update the PK value, a rare thing to do which is logically equivalent to a delete/insert of the PK value), then it would be a good idea to create an appropriate index on the FK of the child tables.

So going back to our demo:

SQL> create index albums_fk_i on albums(artist_id, artist_code);

Index created.

If we now delete a parent row:

SQL> delete artists where id=2 and code = 1;

1 row deleted.

Execution Plan
----------------------------------------------------------
Plan hash value: 898601404

-----------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | DELETE STATEMENT   |              |     1 |    26 |     0   (0)| 00:00:01 |
|   1 |  DELETE            | ARTISTS      |       |       |            |          |
|*  2 |   INDEX UNIQUE SCAN| SYS_C0010352 |     1 |    26 |     0   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

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

   2 - access("ID"=2 AND "CODE"=1)

Statistics
----------------------------------------------------------
          0  recursive calls
          8  db block gets
          1  consistent gets
          0  physical reads
          0  redo size
        864  bytes sent via SQL*Net to client
        839  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
          1  rows processed

We notice the number of consistent gets has dropped dramatically from the previous 3358. So no expensive FTS of the child table and none of the locking issues previously experienced.

But what if the index had the columns in a different order to that specified in the constraints:

SQL> rollback;

Rollback complete.

SQL> drop index albums_fk_i;

Index dropped.

SQL> create index albums_fk_i on albums(artist_code, artist_id);

Index created.

SQL> delete artists where id=2 and code = 1;

1 row deleted.

Execution Plan
----------------------------------------------------------
Plan hash value: 898601404

-----------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | DELETE STATEMENT   |              |     1 |    26 |     0   (0)| 00:00:01 |
|   1 |  DELETE            | ARTISTS      |       |       |            |          |
|*  2 |   INDEX UNIQUE SCAN| SYS_C0010352 |     1 |    26 |     0   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

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

   2 - access("ID"=2 AND "CODE"=1)

Statistics
----------------------------------------------------------
          0  recursive calls
          8  db block gets
          1  consistent gets
          0  physical reads
          0  redo size
        864  bytes sent via SQL*Net to client
        839  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
          1  rows processed

Not a problem. The index contains all the columns of interest and can still be effectively used to quickly check for the existence of the deleted parent value.

What if the index had additional columns defined ?

SQL> rollback;

Rollback complete.

SQL> drop index albums_fk_i;

Index dropped.

SQL> create index albums_fk_i on albums(artist_code, artist_id, album_name);

Index created.

SQL> delete artists where id=2 and code = 1;

1 row deleted.

Execution Plan
----------------------------------------------------------
Plan hash value: 898601404

-----------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | DELETE STATEMENT   |              |     1 |    26 |     0   (0)| 00:00:01 |
|   1 |  DELETE            | ARTISTS      |       |       |            |          |
|*  2 |   INDEX UNIQUE SCAN| SYS_C0010352 |     1 |    26 |     0   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

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

   2 - access("ID"=2 AND "CODE"=1)

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          1  consistent gets
          0  physical reads
          0  redo size
        864  bytes sent via SQL*Net to client
        839  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
          1  rows processed

Again not a problem. As the leading columns contain the FK columns of interest, Oracle can still effectively find the location within the index where the deleted value would be found if it existed in the child table.

So any index in which all the FK columns match the leading columns of the index would suffice.

And it’s really quite easy to create a quick demo to test this all out :)

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