Message-ID: <432c75b0-be5f-4836-b46b-98c505eaad28@postgrespro.ru>
Date: Fri, 7 Mar 2025 00:18:52 +0300
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Subject: Re: explain analyze rows=%.0f
To: Robert Haas <robertmhaas@gmail.com>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: 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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From: Alena Rybakina <a.rybakina@postgrespro.ru>
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On 06.03.2025 17:13, Robert Haas wrote:
> On Thu, Mar 6, 2025 at 8:30 AM Matthias van de Meent
> <boekewurm+postgres@gmail.com> wrote:
>> On Thu, 6 Mar 2025 at 14:18, Alena Rybakina <a.rybakina@postgrespro.ru> wrote:
>>> 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 = 500/999
>> ~=) 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 here!

To be honest, I initially took it as the total number of tuples and 
couldn't figure out for myself how to interpret the result - 0 tuples or 
1 tuple in the end. Maybe it wasn't quite correct to perceive it that 
way, but Matthias's explanation helped me figure out the reason why such 
a strange result was obtained, although it's not usual to see it.

-- 
Regards,
Alena Rybakina
Postgres Professional