Friday, October 12, 2007

Flushing a single cursor

In a very intensive OLTP environment plan stability is of utmost importance as a single sub optimal query can bring the system down to its knees.

Using sql outlines and sql profiles is always not possible if you have an application with more
than a million lines of code and any one of the thousands of sql statements could have a plan flip.

In 11g there is a new procedure in the DBMS_SHARED_POOL package which helps you flush out a single cursor.
There is a typo in the syntax portion of the documentation in
which I will try to get corrected in the next week.

PURGE Procedure
This procedure purges the named object or specified heap(s) of the object.


name VARCHAR2,

Table 118-4 KEEP Procedure Parameters

Parameter Description
name Name of the object to keep.
The value for this identifier is the concatenation of the address and hash_value columns from the v$sqlarea view. This is displayed by the SIZES procedure.

Currently, TABLE and VIEW objects may not be kept.

flag (Optional) If this is not specified, then the package assumes that the first parameter is the name of a package/procedure/function and resolves the name.
Set to 'P' or 'p' to fully specify that the input is the name of a package/procedure/function.

Set to 'T' or 't' to specify that the input is the name of a type.

Set to 'R' or 'r' to specify that the input is the name of a trigger.

Set to 'Q' or 'q' to specify that the input is the name of a sequence.

In case the first argument is a cursor address and hash-value, the parameter should be set to any character except 'P' or 'p' or 'Q' or 'q' or 'R' or 'r' or 'T' or 't'.

heaps Heaps to be purged. For example, if heap 0 and heap 6 are to be purged:
1<<0 | 1<<6 => hex 0x41 => decimal 65, so specify heaps =>65.Default is 1, that is, heap 0 which means the whole object would be purged

This feature was introduced via the fix in bug 5614566 and I actually know a customer who has this applied on top of


SQL> exec dbms_shared_pool.purge('00000003DE576D40,353632309','C',65); ==> purge heap 0 and heap 6

PL/SQL procedure successfully completed.

This would actually not work against a cursor which is currently executing.(pinned)

Session 1:
Do a massive Merge Join Cartesian
select * from dba_objects a, dba_objects b, dba_objects c;

Session 2:
Identify the sql address and hash value and try to purge the cursor..

exec dbms_shared_pool.purge('00000003DE825198,3691928467','C',65); ==> This hangs

and this session is waiting on "cursor: pin X" requesting an exclusive mutex pin for the cursor object whilst it has already been pinned by session 1

Session 3
select event,p1,p2 from v$session where username='SYS' and type='USER';
----------------------------------------- ---------- ----------
cursor: pin X 3691928467 1

The p1 value here is the Hash value of the cursor we are trying to flush.

From the short stack of the process which is executing the purge API a function called kxsPurgeCursor is called which would try to take a mutex (since _kks_use_mutex_pin is TRUE by default)
The purge completes only after you cancel the sql in session 1 and exit from the same
or kill the session executing the sql.


Alex Gorbachev said...

Thanks Fairlie! I just checked note 457309.1 and it works on 10.2 indeed.

Anonymous said...

section 3 :
select event,p1,p2 from v$session where schemaname='SYS' and type='USER';

Anonymous said...

in section 3 : schemaname not username for request submited by 'normal' users :
select event,p1,p2 from v$session where schemaname='SYS' and type='USER';