From: Midge Brown <midgems@sbcglobal.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: slow query, different plans
Date: Mon, 6 Aug 2012 11:36:43 -0700
Message-ID: <6553A486C90148479A9908834FD513DC@BERNICE> (raw)
References: <9CF5A680782142D794A085320949FED4@BERNICE>
<25289.1344061602@sss.pgh.pa.us>
----- Original Message -----
From: Tom Lane
To: Midge Brown
Cc: pgsql-performance@postgresql.org
Sent: Friday, August 03, 2012 11:26 PM
Subject: Re: [PERFORM] slow query, different plans
"Midge Brown" <midgems@sbcglobal.net> writes:
> I'm having a problem with a query on our production server, but not on a laptop running a similar postgres version with a recent backup copy of the same table. I tried reindexing the table on the production server, but it didn't make any difference. Other queries on the same table are plenty fast.
Reindexing won't help that. The problem is a bad statistical estimate;
it thinks there are about 700 rows with applies2 = 256, when there's
really only one. That means the "fast" plan is a lot faster than the
planner gives it credit for, and conversely the "slow" plan is a lot
slower than the planner is expecting. Their estimated costs end up
nearly the same, which makes it a bit of a chance matter which one is
picked --- but the true costs are a lot different. So you need to fix
that rowcount estimate. Raising the stats target for the table might
help.
regards, tom lane
--
I added "and ts is not null" to the query and the planner came back with .102 ms. The problem area in production went from a 10 second response to < 1 second.
Thanks for the responses.
-Midge=
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Subject: Re: slow query, different plans
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