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From: Greg Sabino Mullane <htamfids@gmail.com>
Date: Tue, 1 Oct 2024 10:39:11 -0400
Message-ID: <CAKAnmmJ6wyM2egwebC_R_=H-0SGP7QPzOKepz_=2c=1x9ekiYA@mail.gmail.com>
Subject: Re: Reading execution plan - first row time vs last row time
To: =?UTF-8?B?UGVjc8O2ayBKw6Fu?= <jan.pecsok@profinit.eu>
Cc: "pgsql-general@lists.postgresql.org" <pgsql-general@lists.postgresql.org>
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On Tue, Oct 1, 2024 at 9:53=E2=80=AFAM Pecs=C3=B6k J=C3=A1n <jan.pecsok@pro=
finit.eu> wrote:

> We see significant difference in explain analyze Actual time in the first
> line of execution plan and Execution time in the last line of execution
> plan. What can be the reason?
>
>
>
> For example, first line of execution plan:
>
> Gather  (cost=3D1038.49..257627100.42 rows=3D7943222226 width=3D104) (act=
ual
> time=3D531.925..3673159.806 rows=3D7943221053 loops=3D1)
>

It takes an hour for Postgres to gather up 7.9 BILLION (!) rows, but then
it is going to take additional time to do something with all of those rows.
That's where the rest of your time is going. Seeing your actual query would
be a help. If this is a CTAS (Create Table As Select), then you have an
hour for the 'Select' part, and the rest of your time is in the 'Create
Table' part.

Cheers,
Greg

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<div dir=3D"ltr"><div dir=3D"ltr">On Tue, Oct 1, 2024 at 9:53=E2=80=AFAM Pe=
cs=C3=B6k J=C3=A1n &lt;<a href=3D"mailto:jan.pecsok@profinit.eu">jan.pecsok=
@profinit.eu</a>&gt; wrote:<br></div><div class=3D"gmail_quote"><blockquote=
 class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px so=
lid rgb(204,204,204);padding-left:1ex"><div class=3D"msg-141682381196352448=
1">


<div lang=3D"CS" style=3D"overflow-wrap: break-word;">
<div class=3D"m_-3069553381313727891WordSection1">
<p class=3D"MsoNormal">We see significant difference in explain analyze Act=
ual time in the first line of execution plan and Execution time in the last=
 line of execution plan. What can be the reason?<br></p>
<p class=3D"MsoNormal"><u></u>=C2=A0<u></u></p>
<p class=3D"MsoNormal">For example, first line of execution plan:<u></u><u>=
</u></p>
<p class=3D"MsoNormal">Gather=C2=A0 (cost=3D1038.49..257627100.42 rows=3D79=
43222226 width=3D104) (actual time=3D531.925..3673159.806 rows=3D7943221053=
 loops=3D1)</p></div></div></div></blockquote><div><br></div><div>It takes =
an hour for Postgres to gather up 7.9 BILLION (!) rows, but then it is goin=
g to take additional time to do something with all of those rows. That&#39;=
s where the rest of your time is going. Seeing your actual query would be a=
 help. If this is a CTAS (Create Table As Select), then you have an hour fo=
r the &#39;Select&#39; part, and the rest of your time is in the &#39;Creat=
e Table&#39; part.</div><div><br></div><div>Cheers,</div><div>Greg</div><di=
v><br></div></div></div>

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