From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jonathan Hoover <jhoover@yahoo-inc.com>
Cc: pgsql-admin@postgresql.org <pgsql-admin@postgresql.org>
Subject: Re: Interesting Query Performance Question
Date: Sat, 06 Nov 2010 11:59:48 -0400
Message-ID: <20024.1289059188@sss.pgh.pa.us> (raw)
In-Reply-To: <68C107706843A54BA7D9CAEF123ADF05010346CAE6@SP2-EX07VS03.ds.corp.yahoo.com>
References: <68C107706843A54BA7D9CAEF123ADF05010346CAE6@SP2-EX07VS03.ds.corp.yahoo.com>
"Jonathan Hoover" <jhoover@yahoo-inc.com> writes:
> [ poor performance with NOT IN ]
> Query E then is apparently the way to go, but shouldn't there be a way
> to get the query planner to take these steps itself? If A had ever
> finished, I'd sure like to have seen an EXPLAIN ANALYZE on it.
Well, just an EXPLAIN would have told you what the plan was like.
What I suspect was happening was that your manipulations of the query
altered the planner's estimate of the number of rows in the NOT IN's
subquery, causing it to pick (or not) a hash-table-based implementation
of NOT IN. The hashed approach is a lot faster but requires the
subquery's output to fit in work_mem.
In general, NOT IN is hard to optimize because of its weird behavior
for NULLs. I'd suggest looking into converting the query to use an
EXISTS instead.
regards, tom lane
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To: pgsql-admin@postgresql.org
Cc: tgl@sss.pgh.pa.us, jhoover@yahoo-inc.com
Subject: Re: Interesting Query Performance Question
In-Reply-To: <20024.1289059188@sss.pgh.pa.us>
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