Thread: 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
Chris Rimmer <chrisr@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
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 <tgl@sss.pgh.pa.us> wrote:
Chris Rimmer <chrisr@we7.com> writes:That's pretty weird. What else is being done to that sequence? Is it
> 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)....
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