Is this a PDB running inside a CDB, or is it a non-PDB database ?
Have you checked the execution plan for the query.
Running on a PDB inside a CDB (which is all I can get hands on at the moment) I
can see that there's a cross database call which causes parallel servers to
start up. It's possible that the the issue with cursor pinning is a side effect
of this parallelism; when I ran the query it was one of my PX slaves that had
happened to wait on 'cursor: pin S wait on X'.
A quick check on ASH to see if the parallelism is a problem would be to split
the counts for the wait into one ones where qc_session_id was null (the query
coordinators) and qc_session_id is not null (PX slaves).
I don't know what the parallelism is for - but possibly the effect is made
worse by the presence of RAC - perhaps some of the slaves are being allocated
remotely when ideally they should be allocated locally.
The other problem, of course, is that the data is likely to be extremely skewed
for a few owners - so the optimizer probably does a lot of work calculating
inflection points (my plan show 6 possible inflection points). Have you checked
the data dictionary to see if the stats are reasonably accurate, and whether
you could create some column groups that might help the optimizer get better
estimates of cardinality. Anything which reduces actual optimisation time
might help.
Regards
Jonathan Lewis
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf
of Chris Stephens <cstephens16@xxxxxxxxx>
Sent: 29 June 2018 15:26:23
To: oracle-l
Subject: cursor: pin S wait on X querying data dictionary
3-node RAC 12.2 database on Centos7
We have an application which makes use of SQLAlchemy that is suffering mightily
from "cursor: pin S wait on X" executing the following SQL:
SELECT col.column_name, col.data_type, col.char_length,
col.data_precision, col.data_scale, col.nullable,
col.data_default, com.comments
FROM all_tab_columns col
LEFT JOIN all_col_comments com
ON col.table_name = com.table_name
AND col.column_name = com.column_name
AND col.owner = com.owner
WHERE col.table_name = :table_name
AND col.owner = :owner ORDER BY col.column_id
The application will launch ~200 concurrent sessions on startup. There is a
long initial period where many of them are waiting on the event then things
start clearing up and processing proceeds as expected. There are several
different plan hash values for the SQL, one of which didn't seem to suffer from
the issue so I created a sql baseline and crossed my fingers. I've collected
dictionary statistics as well but that made no difference.
While I do my own searching, does any one have any suggestions on how to remove
the delays?
Here is some ASH data showing the issue:
SQL> @ashtop sql_id,event "sql_id='0rz9dn5v75czn'" "TIMESTAMP'2018-06-28
18:45:00'" "TIMESTAMP'2018-06-29 00:00:00'"
Total
Seconds AAS %This SQL_ID EVENT
FIRST_SEEN LAST_SEEN DIST_SQLEXEC_SEEN
--------- ------- ------- -------------
---------------------------------------- -------------------
------------------- -----------------
75469 4.0 84% | 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:34 2018-06-28 20:38:54 1
6441 .3 7% | 0rz9dn5v75czn library cache lock
2018-06-28 20:11:41 2018-06-28 20:38:36 1
6243 .3 7% | 0rz9dn5v75czn kksfbc child completion
2018-06-28 20:11:44 2018-06-28 20:38:43 1
1592 .1 2% | 0rz9dn5v75czn
2018-06-28 20:11:34 2018-06-28 20:38:55 105
182 .0 0% | 0rz9dn5v75czn cursor: pin S
2018-06-28 20:12:34 2018-06-28 20:38:34 1
133 .0 0% | 0rz9dn5v75czn library cache: mutex X
2018-06-28 20:12:09 2018-06-28 20:38:32 1
18 .0 0% | 0rz9dn5v75czn enq: PS - contention
2018-06-28 20:13:17 2018-06-28 20:38:55 18
15 .0 0% | 0rz9dn5v75czn cursor: pin X
2018-06-28 20:13:41 2018-06-28 20:38:36 1
select snap_id, end_interval_time
8 .0 0% | 0rz9dn5v75czn PGA memory operation
2018-06-28 20:15:24 2018-06-28 20:38:44 1
2 .0 0% | 0rz9dn5v75czn PX Deq: Join ACK
2018-06-28 20:13:26 2018-06-28 20:32:12 2
1 .0 0% | 0rz9dn5v75czn row cache read
2018-06-28 20:35:18 2018-06-28 20:35:18 1
11 rows selected.
SQL> @ashtop session_id,sql_id,event "sql_id='0rz9dn5v75czn'"
"TIMESTAMP'2018-06-28 18:45:00'" "TIMESTAMP'2018-06-29 00:00:00'"
Total
Seconds AAS %This SESSION_ID SQL_ID EVENT
FIRST_SEEN LAST_SEEN DIST_SQLEXEC_SEEN
--------- ------- ------- ---------- -------------
---------------------------------------- -------------------
------------------- -----------------
934 .0 1% | 2065 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:34 2018-06-28 20:37:03 1
925 .0 1% | 1459 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:34 2018-06-28 20:35:26 1
917 .0 1% | 613 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:13:26 2018-06-28 20:38:53 1
893 .0 1% | 4360 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:34 2018-06-28 20:38:45 1
873 .0 1% | 973 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:44 2018-06-28 20:38:26 1
861 .0 1% | 3276 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:44 2018-06-28 20:38:44 1
854 .0 1% | 3877 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:34 2018-06-28 20:38:52 1
828 .0 1% | 4602 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:44 2018-06-28 20:38:50 1
827 .0 1% | 2547 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:34 2018-06-28 20:38:47 1
826 .0 1% | 491 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:44 2018-06-28 20:38:44 1
823 .0 1% | 251 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:15:06 2018-06-28 20:38:51 1
818 .0 1% | 2790 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:15:06 2018-06-28 20:38:35 1
798 .0 1% | 124 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:44 2018-06-28 20:32:06 1
761 .0 1% | 129 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:34 2018-06-28 20:38:45 1
745 .0 1% | 6 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:44 2018-06-28 20:38:51 1
734 .0 1% | 738 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:44 2018-06-28 20:37:01 1
731 .0 1% | 123 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:13:24 2018-06-28 20:38:37 1
730 .0 1% | 4117 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:11:34 2018-06-28 20:38:43 1
726 .0 1% | 1216 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:17:17 2018-06-28 20:38:52 1
710 .0 1% | 3 0rz9dn5v75czn cursor: pin S wait on X
2018-06-28 20:16:46 2018-06-28 20:36:23 1
20 rows selected.
SQL> select distinct(plan_hash_value) from dba_hist_sqlstat where sql_id =
'0rz9dn5v75czn' and snap_id >= 2378;
PLAN_HASH_VALUE
---------------
2901843923
1846450148
0
SQL> l
1 select plan_name, last_executed, enabled, accepted, fixed, executions
2 from dba_sql_plan_baselines
3* where sql_text like '%col.column_name, col.data_type, col.char_length%'
SQL> /
PLAN_NAME LAST_EXECUTED ENABLED ACCEPTED FIXED EXECUTIONS
---------------------------------------- -------------------- ----------
---------- ---------- ----------
SQL_PLAN_78xa0f27qf69ye98653aa YES YES YES 38562
Anyone have any ideas?
Thanks as always!
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