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Re: Plan weirdness. A sort produces more rows than the node beneath it - Mailing list pgsql-performance

From Jeff Janes
Subject Re: Plan weirdness. A sort produces more rows than the node beneath it
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Msg-id CAMkU=1xmXPZYmMqrxKDoYZzGcweiYcpR5dMbV-jWjQOdoF7Bmg@mail.gmail.com
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In response to Plan weirdness. A sort produces more rows than the node beneath it  (Dane Foster <studdugie@gmail.com>)
Responses Re: Plan weirdness. A sort produces more rows than the node beneath it  (Dane Foster <studdugie@gmail.com>)
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On Fri, Aug 4, 2023 at 11:00 AM Dane Foster <studdugie@gmail.com> wrote:
Hello,

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?

This is normal for a merge join.  For every tie in the first input, the qualifying part of the 2nd input must be rescanned, and the rows are tallied again (in the sort node) each time they are rescanned. 
 
Cheers,

Jeff

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