cursor: pin S wait on X

I had a little excitement with the cursor: pin S wait on X wait event when a client’s website came grinding to a halt.

The first thing I checked was blocking sessions…

col username form a12
col event form a25 word_wrapped
col blocking_session head "BLOCKING|SID"
select sid, username, event, blocking_session,
seconds_in_wait, wait_time
from v$session
where state = 'WAITING'
and wait_class != 'Idle'
/

…but there were none (i.e. the BLOCKING_SESSION column was empty). There were, however, a large number of sessions engaged in cursor: pin S wait on X waits.
I took down a few SIDs and ran this query to find out what SQL they were executing:

col sql_text form a150 word_wrap
col child_number head CH# for 999
select
sid,  hash_value,
child_number,
sql_text
from v$sql
where hash_value in (select sql_hash_value from v$session where sid in (&sid))
/

They were all executing the same PL/SQL package. I checked out that package–


select status,
to_char(last_ddl_time, 'yyyy-mm-dd hh24:mi') last_ddl_time
from dba_objects
where upper(owner)=upper('&own')
and upper(object_name) = upper('&name')
/

…and discovered that it had last been modified 15 minutes ago. At that point the site’s DBA came online and he informed me that he had recompiled the package and then had been called away to an unrelated emergency.

Hmm, rolling out untested code on a production server? Grrr… Well, no time for that now; we had to get the website unstuck. He fixed the code and recompiled it, but this did not release those cursor: pin S wait on X waits.

Instead of killing each session one by one (there were dozens of them), I identified the root blocker, whose SID is embedded in the p2raw column of v$session:


select p2raw, count(*)
from v$session
where event = 'cursor: pin S wait on X'
group by p2raw
/
P2RAW            COUNT(*)
---------------- --------
0000028100000000 76
000001EF00000000  3

I went for 0000028100000000 since it had the most sessions. The SID of the blocker is embedded in the first 8 bytes of that string– 281 (hexadecimal). Converted to decimal, the SID is 641.We killed that session…

select serial# from v$session where sid=641;
17938
alter system kill session '641,17938';

…and all the cursor: pin S wait on X waits evaporated and the site sprang to life.
So obviously, this was a blocking situation. Why didn’t my original query, the one that checked for blocking sessions, identify any blockers?

It’s because of a bug in Oracle 10g: the v$session.blocking_session column does not get populated in the case of cursor: pin S wait on X waits. This has apparently been fixed in 11g.

There are a number of bugs pertaining to this wait event. They didn’t apply here, since our smoking gun was our own buggy code. Check out Asif Momen‘s blog for further references.

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