From: Tom Lane <tgl@sss.pgh.pa.us>
To: Scott Rankin <srankin@motus.com>
cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Queries intermittently slow
In-reply-to: <2A061A33-9E7C-44ED-A8A4-411FE128A1C9@motus.com>
References: <2A061A33-9E7C-44ED-A8A4-411FE128A1C9@motus.com>
Comments: In-reply-to Scott Rankin <srankin@motus.com>
 message dated "Wed, 06 Jan 2016 14:58:05 +0000"
Date: Wed, 06 Jan 2016 10:19:15 -0500
Message-ID: <25400.1452093555@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

Scott Rankin <srankin@motus.com> writes:
> I’m 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 I’m trying to trace the reason.  I’ve 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


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