Re: Queries intermittently slow

Scott Rankin <srankin(at)motus(dot)com> writes:
> Im trying to track down why some queries on my database system are intermittently much slower than usual. I have some queries that run, on average, 2-3ms, and they run at a rate of about 10-20 queries/second. However, every 3-5 seconds, one of the queries will be 500-100ms. This is making the average query time turn out to be closer to 20ms, with a very large standard deviation.

> This happens to a number of otherwise very fast queries, and Im trying to trace the reason. Ive turned on lock logging and checkpoint logging, and this behavior happens whether or not a checkpoint is occurring. There are no lock waits happening in the system either.

I doubt you've proved that --- log_lock_waits will only report on waits
longer than deadlock_timeout, which you don't appear to have changed from
its default of 1 sec. If you're trying to capture events that last a few
hundred msec, you're going to need to reduce deadlock_timeout to maybe
100ms.

It would help to know more about what the queries are, too. The cause
might be something like GIN index pending-list cleanup but we can't tell
on the basis of this much info.

regards, tom lane