High Executions Of Statement "delete from smon_scn_time..." (文档 ID 375401.1)

A delete from smon_scn_time is performing excessive gets and executions as viewed from AWR report:

Buffer Gets Executions Gets per Exec %Total CPU Time (s) Elapsed Time (s)
205,501,888 30,508 6,736.00 54.05 9,733.97 61,180.96  delete from smon_scn_time where...

The AWR report shows the following SQL with  an excessive amount of executions:

delete from smon_scn_time where thread=0 and time_mp = (select min(time_mp) from smon_scn_time where thread=0);

Cause

There are inconsistencies between the indexes and table smon_scn_time.

The delete statement deletes the oldest rows from smon_scn_time to clear space for new rows.  SMON wakes up every 5 minutes and checks how many on-disk mappings we have--the max is 144000.

The new mappings are then added for the last period (since SMON last updated), and if this is over 144000, SMON will then issue the delete statement:

delete from smon_scn_time where thread=0 and time_mp = (select min(time_mp) from smon_scn_time where thread=0)

There will be an execution of this each time SMON wakes to update smon_scn_time, and if one deletion does not free enough mappings, then there will be multiple executions.

What happens is due to the inconsistency between the table and indexes the delete returns zero rows; so the delete statement is executed continuously to reduce the smon_scn_time below the maximum 14400 mappings.

When table smon_scn_time is analyzed we see the inconsistency:

SQL> analyze table smon_scn_time validate structure cascade;

analyze table smon_scn_time validate structure cascade
*
ERROR at line 1 :
ORA-01499: table/Index Cross Reference Failure - see trace file

Solution

To implement the solution, please execute the following steps:

1. Ensure you have a usable backup in case of failures

2. Drop and recreate the indexes on table smon_scn_time

connect / as sysdba
drop index smon_scn_time_scn_idx;
drop index smon_scn_time_tim_idx;
create unique index smon_scn_time_scn_idx on smon_scn_time(scn);
create unique index smon_scn_time_tim_idx on smon_scn_time(time_mp);
analyze table smon_scn_time validate structure cascade;