From: Tom Lane <tgl@sss.pgh.pa.us>
To: Dane Foster <studdugie@gmail.com>
Cc: psql-performance <pgsql-performance@postgresql.org>
Subject: Re: Plan weirdness. A sort produces more rows than the node beneath it
Date: Fri, 04 Aug 2023 11:31:57 -0400
Message-ID: <3661332.1691163117@sss.pgh.pa.us> (raw)
In-Reply-To: <CA+WxinK_PeQBeZ-SWVqheHNjyjtD3QGZ5gZSzVKxHRYgUG-ktw@mail.gmail.com>
References: <CA+WxinKHxRUgDwbUfsKNiq5MXn8kRFayo0KqZzTmxXG93BbK4g@mail.gmail.com>
<3658472.1691161821@sss.pgh.pa.us>
<CA+WxinK_PeQBeZ-SWVqheHNjyjtD3QGZ5gZSzVKxHRYgUG-ktw@mail.gmail.com>
Dane Foster <studdugie@gmail.com> writes:
>> If the sort is the inner input to a merge join, this could reflect
>> mark-and-restore rescanning of the sort's output. Are there a
>> whole lot of duplicate keys on the merge's other side?
> Yes. The course_id column's values repeat a LOT on the merge side.
Hmm. The planner should avoid using a merge join if it knows that
to be true. Maybe analyze'ing that table would prompt it to use
some other join method?
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, studdugie@gmail.com
Subject: Re: Plan weirdness. A sort produces more rows than the node beneath it
In-Reply-To: <3661332.1691163117@sss.pgh.pa.us>
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