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From: Robert Haas <robertmhaas@gmail.com>
Date: Thu, 6 Mar 2025 09:13:45 -0500
Message-ID: <CA+TgmobD4_fz4JnGkYey_2rsXi5mxucDPR2ot6rX9OoHBw+qhg@mail.gmail.com>
Subject: Re: explain analyze rows=%.0f
To: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Alena Rybakina <a.rybakina@postgrespro.ru>, 
	Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>, Tom Lane <tgl@sss.pgh.pa.us>, 
	Andrei Lepikhov <lepihov@gmail.com>, pgsql-hackers@lists.postgresql.org, 
	Guillaume Lelarge <guillaume@lelarge.info>, Matheus Alcantara <matheusssilv97@gmail.com>
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On Thu, Mar 6, 2025 at 8:30=E2=80=AFAM Matthias van de Meent
<boekewurm+postgres@gmail.com> wrote:
> On Thu, 6 Mar 2025 at 14:18, Alena Rybakina <a.rybakina@postgrespro.ru> w=
rote:
> >
> > Hi! I got a query plan with a strange number of rows. Could you please
> > help me understand it?
> >
> > To be honest I can't understand why 0.50 number of rows here?
>
> Because the scan matched only ~(500 rows over 999 iterations =3D 500/999
> ~=3D) 0.50 rows for every loop, on average, for these plan nodes:

This is a good and correct explanation, but I'm VERY curious to hear
more from Alena. Like, Tom expressed the concern before we did this
that the fractional digits would confuse people, and the fact that
someone who is a regular participant on this mailing list was one of
the people confused gives credence to that concern. But I want to know
what exactly Alena found (or finds) confusing here. The Nested Loop
executes 999 times, so perhaps Alena thought that 0.50 was the TOTAL
number of rows across ALL of those executions rather than the AVERAGE
number of rows per execution? Because then 0.50 would indeed be a very
surprising result. Or maybe she just didn't realize that part of the
plan executed 999 times? Or something else?

Alena, if you're willing, please elaborate on what you think is confusing h=
ere!

--=20
Robert Haas
EDB: http://www.enterprisedb.com