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 <CAB-JLwYCgdiB=trauAV1HN5rAWQdvDGgaaY_mqziN88pBTvqqg@mail.gmail.com>
 <CAOtHd0BJpzox6iW3Lm3Rcut8edeR+4xfW-7M_gWMRdkhp0oRxw@mail.gmail.com>
 <34604.1758921116@sss.pgh.pa.us>
In-Reply-To: <34604.1758921116@sss.pgh.pa.us>
Reply-To: maciek@sakrejda.org
From: Maciek Sakrejda <m.sakrejda@gmail.com>
Date: Fri, 26 Sep 2025 17:31:43 -0700
Message-ID: 
 <CAOtHd0Ame_kKndkjBzKPpCFoy6x3HoYrf0DeAoufT0ykNuDPEg@mail.gmail.com>
Subject: Re: V18 change on EXPLAIN ANALYZE
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: maciek@sakrejda.org, Robert Haas <robertmhaas@gmail.com>,
	Marcos Pegoraro <marcos@f10.com.br>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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On Fri, Sep 26, 2025 at 2:12=E2=80=AFPM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Maciek Sakrejda <m.sakrejda@gmail.com> writes:
> > The page you link says
>
> >     In some query plans, it is possible for a subplan node to be
> > executed more than once. For example, the inner index scan will be
> > executed once per outer row in the above nested-loop plan. In such
> > cases, the loops value reports the total number of executions of the
> > node, and the actual time and rows values shown are averages
> > per-execution. This is done to make the numbers comparable with the
> > way that the cost estimates are shown. Multiply by the loops value to
> > get the total time actually spent in the node. In the above example,
> > we spent a total of 0.030 milliseconds executing the index scans on
> > tenk2.
>
> > in the second paragraph after the example in this section. Do you
> > think that's not sufficiently clear?
>
> It's not wrong, but it feels a little incomplete now.  Maybe change
> the last two sentences to
>
> Multiply by the loops value to get the total time actually spent in
> the node and the total number of rows processed by the node across all
> executions. In the above example, we spent a total of 0.030
> milliseconds executing the index scans on tenk2, and they handled a
> total of 10 rows.
>
> A bigger gap in perform.sgml is that it doesn't address parallel
> query cases at all AFAICS.  I think that was one of the main drivers
> of this change, so it feels a little sad that it's not covered here.

Fair point. I included your proposed change and took a stab at briefly
covering parallelism in the attached (admittedly, my understanding of
how that works is a little shaky, so apologies if I'm way off on some
of this).

However, to get a parallel query in the regression database (I chose
EXPLAIN ANALYZE SELECT * FROM tenk2), I had to change some settings:

SET min_parallel_table_scan_size =3D 0;
SET parallel_tuple_cost =3D 0;
SET parallel_setup_cost =3D 0;

Should I mention that in the example? Or should I generate a bigger
table so using these is not necessary? If we say nothing and use the
example, I think it may be confusing if someone wants to use the
example as a starting point for their own exploration of how this
works. Or is there a better query that works out of the box and does
not need changes to the settings?

It also seems like the EXPLAIN ANALYZE section is getting a little
unwieldy. Should we subdivide it, or is this still okay?

Thanks,
Maciek

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