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From: Maciek Sakrejda <[email protected]>
To: Marcos Pegoraro <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: V18 change on EXPLAIN ANALYZE
Date: Fri, 26 Sep 2025 14:01:37 -0700
Message-ID: <CAOtHd0BJpzox6iW3Lm3Rcut8edeR+4xfW-7M_gWMRdkhp0oRxw@mail.gmail.com> (raw)
In-Reply-To: <CAB-JLwYCgdiB=trauAV1HN5rAWQdvDGgaaY_mqziN88pBTvqqg@mail.gmail.com>
References: <CAB-JLwYCgdiB=trauAV1HN5rAWQdvDGgaaY_mqziN88pBTvqqg@mail.gmail.com>

On Fri, Sep 26, 2025 at 1:34 PM Marcos Pegoraro <[email protected]> wrote:
> Robert Hass committed ddb17e387aa28d61521227377b00f997756b8a27, which changed how EXPLAIN indicates fractional rows. That's cool but I have to see that change on sources because it's not explained on DOCs.
>
> One can understand that cost=4.36, that actual time=0.009, but what means rows=10.43 ? Costs and Time having decimals are fine but what is a row and a half ?
>
> So, I don't know exactly how to explain this change but I think it would be good to have a new paragraph on [1] and explain that now when nloops > 1, we now display two digits after the decimal point, rather than none.
>
> [1] - https://www.postgresql.org/docs/current/using-explain.html#USING-EXPLAIN-ANALYZE

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?

Thanks,
Maciek

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