Index Monitoring and Foreign Keys – Caution (Kid A)

One of the questions asked of indexes is are they actually being used or are they sitting there looking pretty not being used, wasting valuable resources and storage.

One possible method of determining whether an index is being used is to set the MONITORING USAGE attribute of the index. This will place the index in the v$object_usage view and will flag the index as being used when indeed the Cost Based Optimizer decides to “use” the index in an execution plan.

For example:

SQL> create table daddy (id number constraint daddy_pk primary key, name varchar2(20));

Table created.

SQL> insert into daddy values (1, ‘BOWIE’);

1 row created.

SQL> insert into daddy values (2, ‘ZIGGY’);

1 row created.

SQL> insert into daddy values (3, ‘THIN WHITE DUKE’);

1 row created.

SQL> commit;

Commit complete.

SQL> alter index daddy_pk monitoring usage;

Index altered.

If we look at the v$object_usage view, we’ll now see an entry for this index, with the USED flag currently set to NO:

SQL> select index_name, monitoring, used from v$object_usage where index_name = ‘DADDY_PK’;

INDEX_NAME   MON USE
------------ --- ---
DADDY_PK     YES NO

If we however make the CBO “use” this index:

SQL> select * from daddy where id = 1;

        ID NAME
---------- --------------------
         1 BOWIE

--------------------------------------------
|Id| Operation                   | Name    |
--------------------------------------------
| 0| SELECT STATEMENT            |         |
| 1|  TABLE ACCESS BY INDEX ROWID| DADDY   |
|*2|   INDEX UNIQUE SCAN         | DADDY_PK|
--------------------------------------------

We now notice that the index is now marked as having been used:

SQL> select index_name, monitoring, used from v$object_usage where index_name = ‘DADDY_PK’;

INDEX_NAME   MON USE
------------ --- ---
DADDY_PK     YES YES

Although it doesn’t give any indication on just how often the index is actually used, if an index is still marked as unused for a period of time, it does provide some indication that the index has not been used during that time and is a candidate for being dropped.

Or does it ?

Unfortunately, the are some cases when an index is being used but monitoring the index doesn’t give any such indication. However dropping such indexes can prove disastrous …

For example, let’s create another table that has a Foreign Key which references this DADDY table:

SQL> create table kiddie (id number, name varchar2(20), fk number, constraint kiddie_fk foreign key(fk) references daddy(id));

Table created.

Let’s now populate it with a number of rows:

SQL> insert into kiddie select rownum, ‘MAJOR TOM’, 1 from dual connect by level <= 1000000;

1000000 rows created.

SQL> commit;

Commit complete.

Let’s now create an index on the Foreign Key column and collect some stats:

SQL> create index kiddie_fk_i on kiddie(fk);

Index created.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>’DADDY’, estimate_percent=>null, cascade=>true, method_opt=> ‘FOR ALL COLUMNS SIZE 1′);

PL/SQL procedure successfully completed.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>’KIDDIE’, estimate_percent=>null, cascade=>true, method_opt=> ‘FOR ALL COLUMNS SIZE 1’);

PL/SQL procedure successfully completed.

Let’s now monitor this index on the Foreign Key and see where it may or may not get used:

SQL> alter index kiddie_fk_i monitoring usage;

Index altered.

SQL> select index_name, monitoring, used from v$object_usage where index_name = ‘KIDDIE_FK_I’;

INDEX_NAME   MON USE
------------ --- ---
KIDDIE_FK_I  YES NO

OK, let’s now delete a row from the parent table and look at the statistics to see how many consistent reads were performed:

SQL> delete daddy where id = 2;

1 row deleted.

Statistics
---------------------------
          8 db block gets
         26 consistent gets
          1 physical reads
          1  rows processed

Now, behind the scenes, Oracle indeed used the index on the Foreign Key to determine whether or not the parent row could be safely deleted or whether there were child records that were still referencing the parent row. Note we used just 26 consistent reads.

Let’s see if the monitoring of the index has picked this up …

SQL> select index_name, monitoring, used from v$object_usage where index_name = ‘KIDDIE_FK_I’;

INDEX_NAME   MON USE
------------ --- ---
KIDDIE_FK_I  YES NO

Ooops, this is not good. Is this index really not being used, is it really perhaps safe to drop.

Let’s find out:

SQL> drop index kiddie_fk_i;

Index dropped.

Let’s now delete another parent row and see if we notice a difference in performance:

SQL> delete daddy where id = 3;

1 row deleted.

Statistics
----------------------------
          7  db block gets
       3194  consistent gets
       1599  physical reads
          1  rows processed

Ooops, the consistent gets have jumped up dramatically from 26 to a massive 3194 !! Without the index on the Foreign Key, the only way now for Oracle to check whether it’s OK to delete a parent row is to perform a Full Table Scan on the child table. Not only is this potentially very costly but there could be some nasty locking implications as well.

Index Monitoring only tells you if an index has been used or if it hasn’t been used by the CBO.

It doesn’t however tell you whether an index has actually been used or whether it’s safe to drop the index.

More examples of when it may not be safe to drop a so called unused index to come …