From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jesper Krogh <jesper@krogh.cc>
Cc: pgsql-performance@postgresql.org
Subject: Re: Message queue table..
Date: Fri, 18 Apr 2008 15:27:02 -0400
Message-ID: <7590.1208546822@sss.pgh.pa.us> (raw)
In-Reply-To: <4808DF33.8000008@krogh.cc>
References: <4808DF33.8000008@krogh.cc>
Jesper Krogh <jesper@krogh.cc> writes:
> I have this "message queue" table.. currently with 8m+ records. Picking
> the top priority messages seem to take quite long.. it is just a matter
> of searching the index.. (just as explain analyze tells me it does).
> Limit (cost=0.00..0.09 rows=1 width=106) (actual
> time=245.273..245.274 rows=1 loops=1)
> -> Index Scan using workqueue_job_funcid_priority_idx on job
> (cost=0.00..695291.80 rows=8049405 width=106) (actual
> time=245.268..245.268 rows=1 loops=1)
> Index Cond: (funcid = 4)
> Filter: ((run_after <= 1208442668) AND (grabbed_until <=
> 1208442668) AND ("coalesce" = 'Efam'::text))
> Total runtime: 245.330 ms
Well, what that's doing in English is: scan all the rows with funcid =
4, in priority order, until we hit the first one satisfying the filter
conditions. Apparently there are a lot of low-priority rows that have
funcid = 4 but not the other conditions.
If it's the "coalesce" condition that's the problem, an index on
(funcid, coalesce, priority) --- or (coalesce, funcid, priority) ---
would probably help. I'm not sure there's a simple fix if it's
the other conditions that are really selective.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, jesper@krogh.cc
Subject: Re: Message queue table..
In-Reply-To: <7590.1208546822@sss.pgh.pa.us>
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