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	Wed, 17 Mar 2021 16:47:35 -0400
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Marcin Gozdalik <gozdal@gmail.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Extremely inefficient merge-join
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Comments: In-reply-to Marcin Gozdalik <gozdal@gmail.com>
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Marcin Gozdalik <gozdal@gmail.com> writes:
> Sometimes Postgres will choose very inefficient plan, which involves
> looping many times over same rows, producing hundreds of millions or
> billions of rows:

Yeah, this can happen if the outer side of the join has a lot of
duplicate rows.  The query planner is aware of that effect and will
charge an increased cost when it applies, so I wonder if your
statistics for the tables being joined are up-to-date.

			regards, tom lane