[PERFORM] Select from sequence in slow query log
While investigating some performance issues I have been looking at slow
queries logged to the postgresql.log file. A strange thing that I have
seen is a series of apparently very slow queries that just select from a
sequence. It is as if access to a sequence is blocked for many sessions and
then released as I get log entries like this appearing:
LOG: duration: 23702.553 ms execute unnamed: /* dynamic native SQL
query */ select nextval ('my_sequence') as nextval
LOG: duration: 23673.068 ms execute unnamed: /* dynamic native SQL
query */ select nextval ('my_sequence') as nextval
LOG: duration: 23632.729 ms execute unnamed: /* dynamic native SQL
query */ select nextval ('my_sequence') as nextval
(Many similar lines)
LOG: duration: 3055.057 ms execute unnamed: /* dynamic native SQL query
*/ select nextval ('my_sequence') as nextval
LOG: duration: 2377.621 ms execute unnamed: /* dynamic native SQL query
*/ select nextval ('my_sequence') as nextval
LOG: duration: 743.732 ms execute unnamed: /* dynamic native SQL query
*/ select nextval ('my_sequence') as nextval
The code is being executed via Hibernate, but using
Session.createSQLQuery(), so the SQL above appears in the source as above
(minus the comment) and not as part of any ORM magic. We are using
Postgresql 9.0.
This seems very strange to me. What could cause a sequence to be locked for
such a long time?
The sequence in question has cache set at 1. Would setting this higher make
any difference?
Thanks
Chris
Re: [PERFORM] Select from sequence in slow query log
Chris Rimmer chr...@we7.com writes:
While investigating some performance issues I have been looking at slow
queries logged to the postgresql.log file. A strange thing that I have
seen is a series of apparently very slow queries that just select from a
sequence. It is as if access to a sequence is blocked for many sessions and
then released as I get log entries like this appearing:
LOG: duration: 23702.553 ms execute unnamed: /* dynamic native SQL
query */ select nextval ('my_sequence') as nextval
LOG: duration: 23673.068 ms execute unnamed: /* dynamic native SQL
query */ select nextval ('my_sequence') as nextval
LOG: duration: 23632.729 ms execute unnamed: /* dynamic native SQL
query */ select nextval ('my_sequence') as nextval
(Many similar lines)
That's pretty weird. What else is being done to that sequence? Is it
only the sequence ops that are slow, or does this happen at times when
everything else is slow too? Can you create a reproducible test case?
regards, tom lane
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Re: [PERFORM] Select from sequence in slow query log
It looks like this effect only occurs in the middle of the night when there
is some kind of automated dump process going on and the system is under
higher than normal load. I haven't managed to reproduce them outside of
production, but since these oddities don't seem to show up during normal
operations, I'm not worrying too much about them now.
Thanks
Chris
On 1 June 2012 14:47, Tom Lane t...@sss.pgh.pa.us wrote:
Chris Rimmer chr...@we7.com writes:
While investigating some performance issues I have been looking at slow
queries logged to the postgresql.log file. A strange thing that I have
seen is a series of apparently very slow queries that just select from a
sequence. It is as if access to a sequence is blocked for many sessions
and
then released as I get log entries like this appearing:
LOG: duration: 23702.553 ms execute unnamed: /* dynamic native SQL
query */ select nextval ('my_sequence') as nextval
LOG: duration: 23673.068 ms execute unnamed: /* dynamic native SQL
query */ select nextval ('my_sequence') as nextval
LOG: duration: 23632.729 ms execute unnamed: /* dynamic native SQL
query */ select nextval ('my_sequence') as nextval
(Many similar lines)
That's pretty weird. What else is being done to that sequence? Is it
only the sequence ops that are slow, or does this happen at times when
everything else is slow too? Can you create a reproducible test case?
regards, tom lane
