From: Tom Lane <tgl@sss.pgh.pa.us>
To: Drew Wilson <drewmwilson@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: how to efficiently update tuple in many-to-many relationship?
Date: Mon, 09 Apr 2007 22:13:05 -0400
Message-ID: <16374.1176171185@sss.pgh.pa.us> (raw)
In-Reply-To: <22512E5C-A60F-43E7-A370-257F7AF956FD@gmail.com>
References: <233A9987-ABA5-4491-A304-FDA1F8FCD962@gmail.com>
<14019.1176155022@sss.pgh.pa.us>
<22512E5C-A60F-43E7-A370-257F7AF956FD@gmail.com>
Drew Wilson <drewmwilson@gmail.com> writes:
> Here's the query plan for a SELECT statement that returns 1,207,161
> rows in 6 seconds.
> ...
> And here's the query plan for the UPDATE query that seems to never
> complete. (Execution time > 30 minutes.)
Well, the subplan is certainly the same as before, so it seems there are
two possibilities:
* there's something unreasonably inefficient about the hash join being
used to perform the IN (work_mem too small? inefficient-to-compare
datatype? bad data distribution?)
* the time is actually going into the UPDATE operation proper, or
perhaps some triggers it fires (have you got any foreign keys involving
this table? what's checkpoint_segments set to?)
You could narrow it down by checking the runtime for
select count(*) from translation_pair_data
where translation_pair_id in
(select translation_pair_id from translation_pair_data ...
If that's slow it's the topmost hash join's fault, else we have
to look at the UPDATE's side effects.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, drewmwilson@gmail.com
Subject: Re: how to efficiently update tuple in many-to-many relationship?
In-Reply-To: <16374.1176171185@sss.pgh.pa.us>
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