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Using dbms_shared_pool.purge to remove a single task from the library cache 

Oracle Database Tips by Donald BurlesonJune 1,  2015


Question:  I know that it is possible to flush the entire shared pool, but I need a procedure to flush a specific SQL statement?  Does a procedure exist that will flush the shared pool contents for a specific session?

Answer:  Yes, is it possible to kill any session with the "alter system kill session" syntax, but you can also use the dbms_shared_pool.purge procedure to remove all shared pool contents for a specific cursor session. 

This article by Fairlie Rego notes a method for purging the RAM used by a specific cursor session using the dbms_shared_pool.purge procedure: 

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'.

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 10.2.0.3.

Here is an example is using dbms_shared_pool.purge to remove RAM for a specific cursor from the shared pool library cache:

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';
EVENT P1 P2
----------------------------------------- ---------- ----------
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.


 

 

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