Child cursors not getting purged 11.2.0.2

Question

select * from v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production
SQL_ID                         = 7bz1uuvdrkdbm
ADDRESS                        = 000000031CEBB878
CHILD_ADDRESS                  = 000000031CEBB418
CHILD_NUMBER                   = 0
ROLL_INVALID_MISMATCH          = Y
All child cursors have same ROLL_INVALID_MISMATCH   = Y
I have shared_pool_size=2 GB.
select sql_id,child_number CN,plan_hash_value phv,executions,elapsed_time/executions/1000000 "sec" ,IS_BIND_SENSITIVE SEN,is_bind_aware AWR,IS_SHAREABLE SHA,LAST_LOAD_TIME LOAD_T,last_active_time ACTIVE_T,substr(sql_fulltext,1,40) from v$sql where sql_id='&sql_id' order by last_active_time;
Enter value for sql_id: 7bz1uuvdrkdbm
SQL_ID                 CN        PHV EXECUTIONS        sec S A S LOAD_T                    ACTIVE_T                  SUBSTR(SQL_FULLTEXT,1,40)
7bz1uuvdrkdbm           7   57216169     204160 .043308046 Y N Y 2012-10-19/21:41:52       20.Oct.12/21:03:35        select instrops0_.detail_id as detail41_
7bz1uuvdrkdbm           0   57216169     185981  .04124503 Y N Y 2012-10-20/21:03:44       21.Oct.12/18:02:37        select instrops0_.detail_id as detail41_
7bz1uuvdrkdbm           1   57216169     252623 .043459744 Y N Y 2012-10-21/18:02:38       22.Oct.12/21:32:07        select instrops0_.detail_id as detail41_
7bz1uuvdrkdbm           2   57216169     240260 .043797356 Y N Y 2012-10-22/21:32:35       23.Oct.12/21:16:31        select instrops0_.detail_id as detail41_
7bz1uuvdrkdbm           3   57216169       9331 .047150057 Y N Y 2012-10-23/21:16:40       23.Oct.12/23:33:15        select instrops0_.detail_id as detail41_
7bz1uuvdrkdbm           4   57216169      38965 .045418917 Y N Y 2012-10-23/23:33:25       24.Oct.12/04:18:29        select instrops0_.detail_id as detail41_
select sql_id,loads,loaded_versions,invalidations,executions from v$sql where sql_id='7bz1uuvdrkdbm' order by child_number;
SQL_ID              LOADS LOADED_VERSIONS INVALIDATIONS EXECUTIONS
7bz1uuvdrkdbm          21               1            20     185981
7bz1uuvdrkdbm          17               1            16     252623
7bz1uuvdrkdbm          14               1            13     240260
7bz1uuvdrkdbm          22               1            21       9331
7bz1uuvdrkdbm          11               1            10      45902
7bz1uuvdrkdbm           9               1             8     204160
6 rows selected.I understand that child cursors are created due to object stats update by nightly dbms_stats.gather_schema_stats. (ROLL_INVALID_MISMATCH caused by DBMS_STATS.AUTO_INVALIDATE)
My question is, why Oracle is not purging old child cursors if they are not being used. (For example....Child_number=7 is last used on 20.Oct.12/21:03:35)
Is there any way i can have auto purge them after certain threshold ? Is that not misuse of shared pool memory? I believe rolling invalidation is to prevent hard parse (and latch contention issues) by cursor invalidation in high OLTP system. But if OLTP system is active 24 hr a day, we can still observe latch contention after 5 hr (_optimizer_invalidation_period) ...right ?

Answer 1

Hi,
shared pool housekeeping is not as simple as you imagine it to be. It's not like at any given moment of time there is only one "correct" child cursor, the rest being subject to purging, it's much more complex, and the exact housekeeping algorithms are not accessible to us users. Plus, you only have several child curors; I once had over a hundred and raised a SR to that effect -- the customer support represebtatuve said that cursor sharing mechanisms in Oracle aren't perfect and unless one has thousands of child cursors one shouldn't be worried.
Best regards,
Nikolay