| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Andrei Lepikhov <lepihov(at)gmail(dot)com> |
| Cc: | Ilia Evdokimov <ilya(dot)evdokimov(at)tantorlabs(dot)com>, Matheus Alcantara <matheusssilv97(at)gmail(dot)com>, Guillaume Lelarge <guillaume(at)lelarge(dot)info>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Ibrar Ahmed <ibrar(dot)ahmad(at)gmail(dot)com>, "Gregory Stark (as CFM)" <stark(dot)cfm(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Peter Geoghegan <pg(at)bowt(dot)ie>, vignesh C <vignesh21(at)gmail(dot)com>, "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>, pgsql-hackers(at)lists(dot)postgresql(dot)org |
| Subject: | Re: explain analyze rows=%.0f |
| Date: | 2025-02-11 17:41:26 |
| Message-ID: | CA+Tgmobgkh5CW=4Xpejw9nR6Onv8SZh0nnr+=HmNAFyyHZb0eg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Tue, Feb 11, 2025 at 12:14 PM Andrei Lepikhov <lepihov(at)gmail(dot)com> wrote:
> I support the idea in general, but I believe it should be expanded to
> cover all cases of parameterised plan nodes. Each rescan iteration may
> produce a different number of tuples, and rounding can obscure important
> data.
>
> For example, consider five loops of a scan node: the first loop returns
> nine tuples, and each other - zero tuples. When we calculate the
> average, 9 divided by 5 equals 1.8. This results in an explanation that
> indicates "rows = 1," masking almost 40% of the data.
>
> Now, if we apply the same two loops but have a total of 900,000 tuples,
> then 400,000 masked tuples represent a significant portion of the data.
>
> Moreover, switching to a floating-point type for row explanations in
> each parameterised node would provide a more comprehensive view and add
> valuable information about the parameterisation of the node, which may
> not be immediately apparent.
I agree strongly with all of this. I believe we should just implement
what was agreed here:
https://www.postgresql.org/message-id/21013.1243618236%40sss.pgh.pa.us
Let's just display 2 fractional digits when nloops>1, else 0, and call it good.
--
Robert Haas
EDB: http://www.enterprisedb.com
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