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 11:10:21 -0400
Message-ID: <3658472.1691161821@sss.pgh.pa.us> (raw)
In-Reply-To: <CA+WxinKHxRUgDwbUfsKNiq5MXn8kRFayo0KqZzTmxXG93BbK4g@mail.gmail.com>
References: <CA+WxinKHxRUgDwbUfsKNiq5MXn8kRFayo0KqZzTmxXG93BbK4g@mail.gmail.com>
Dane Foster <studdugie@gmail.com> writes:
> I'm trying to understand a bit of weirdness in a plan output. There is a
> sort node above a sequential scan node where the scan node produces 26,026
> rows yet the sort node above it produces 42,995,408. How is it possible to
> sort more data than you received?
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?
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: <3658472.1691161821@sss.pgh.pa.us>
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