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Date: Mon, 31 Mar 2025 23:59:48 +0300
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Subject: Re: explain analyze rows=%.0f
To: Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>,
 Andrei Lepikhov <lepihov@gmail.com>, pgsql-hackers@lists.postgresql.org,
 Guillaume Lelarge <guillaume@lelarge.info>,
 Matheus Alcantara <matheusssilv97@gmail.com>,
 Alena Rybakina <a.rybakina@postgrespro.ru>
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From: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>
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On 31.03.2025 22:09, Robert Haas wrote:
> Oh, right. I've never really understood why we round off to integers,
> but the fact that we don't allow row counts < 1 feels like something
> pretty important. My intuition is that it probably helps a lot more
> than it hurts, too.


We definitely shouldn’t remove the row counts < 1 check, since there are 
many places in the planner where we divide by rows. This mechanism was 
added specifically to prevent division by zero. Also, allowing rows 
estimates below 1 can sometimes make the planner overly optimistic, 
leading it to prefer cheaper-looking plans that may not perform well in 
practice. For example, choosing a Nested Loop instead of a more 
appropriate Hash Join.

Allowing fractional rows > 1 might help improve planner accuracy in some 
cases, but this needs further study to fully understand the impact.

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.

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    <p><br>
    </p>
    <div class="moz-cite-prefix">On 31.03.2025 22:09, Robert Haas wrote:<span
      style="white-space: pre-wrap">
</span></div>
    <blockquote type="cite"
cite="mid:CA+TgmobMEbWEDsKQ5ezaBEnwcWLcTef5yupx2bOj03aKj1GvRA@mail.gmail.com">
      <pre class="moz-quote-pre" wrap="">Oh, right. I've never really understood why we round off to integers,
but the fact that we don't allow row counts &lt; 1 feels like something
pretty important. My intuition is that it probably helps a lot more
than it hurts, too.
</pre>
    </blockquote>
    <p><br>
    </p>
    <p>We definitely shouldn’t remove the row counts &lt; 1 check, since
      there are many places in the planner where we divide by rows. This
      mechanism was added specifically to prevent division by zero.
      Also, allowing rows estimates below 1 can sometimes make the
      planner overly optimistic, leading it to prefer cheaper-looking
      plans that may not perform well in practice. For example, choosing
      a Nested Loop instead of a more appropriate Hash Join.</p>
    <p>Allowing fractional rows &gt; 1 might help improve planner
      accuracy in some cases, but this needs further study to fully
      understand the impact.</p>
    <p>--<br>
      Best regards,<br>
      Ilia Evdokimov,<br>
      Tantor Labs LLC.<br>
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