To: Tobias Brox <tobixen@gmail.com>
cc: pgsql-performance@postgresql.org
Subject: Re: locking issue on simple selects? 
In-reply-to: <AANLkTikh++7mouBb7cr2C9zMPKtRffVxDK0QFTf16HLo@mail.gmail.com> 
References: <AANLkTikh++7mouBb7cr2C9zMPKtRffVxDK0QFTf16HLo@mail.gmail.com>
Comments: In-reply-to Tobias Brox <tobixen@gmail.com>
	message dated "Wed, 15 Sep 2010 12:05:03 +0200"
Date: Wed, 15 Sep 2010 09:39:47 -0400
Message-ID: <4096.1284557987@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Tobias Brox <tobixen@gmail.com> writes:
> Recently we've frequently encountered issues where some simple selects
> (meaning, selects doing an index lookup and fetching one row) have
> become stuck for several minutes.  Apparently all requests on one
> exact table gets stuck, all requests not related to said table are
> going through without any problems.  According to the pg_stat_activity
> view, all queries getting stuck was read-queries (selects), no updates
> or anything like that (some of the transactions were doing updates
> and/or inserts though).

> The obvious thought seems to be that this is a locking issue ... but
> it doesn't seem so.  For one thing, AFAIK locking shouldn't affect
> selects, only updates?

An exclusive lock will block selects too.  Have you looked into pg_locks
for ungranted lock requests?

			regards, tom lane