From: Antonin Houska <ah@cybertec.at>
To: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
cc: Tom Lane <tgl@sss.pgh.pa.us>,
    Andrew Gierth <andrew@tao11.riddles.org.uk>,
    pgsql-hackers@postgresql.org
Subject: Re: Problems with plan estimates in postgres_fdw
In-reply-to: <5C7928C7.50507@lab.ntt.co.jp>
References: <87pnz1aby9.fsf@news-spur.riddles.org.uk>
 <1175.1533220908@sss.pgh.pa.us> <5BB739CD.6040002@lab.ntt.co.jp>
 <5BBC4140.50403@lab.ntt.co.jp> <5C17A00E.5000005@lab.ntt.co.jp>
 <5C232F39.9060509@lab.ntt.co.jp> <5C25C7B5.2010802@lab.ntt.co.jp>
 <5C518631.5080401@lab.ntt.co.jp> <10994.1549559088@localhost>
 <5C5D778F.3080501@lab.ntt.co.jp> <19433.1549962210@localhost>
 <5C669DFD.30002@lab.ntt.co.jp> <16616.1550234762@localhost>
 <5C6A5FD3.8080102@lab.ntt.co.jp> <30524.1550499671@localhost>
 <5C6BBD7A.1030702@lab.ntt.co.jp> <29190.1550843669@localhost>
 <5C73C403.7080109@lab.ntt.co.jp> <7273.1551438041@localhost>
 <5C7928C7.50507@lab.ntt.co.jp>
Comments: In-reply-to Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
   message dated "Fri, 01 Mar 2019 21:42:47 +0900."
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Date: Fri, 01 Mar 2019 17:14:25 +0100
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Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> wrote:

> (2019/03/01 20:00), Antonin Houska wrote:
> > Etsuro Fujita<fujita.etsuro@lab.ntt.co.jp>  wrote:

> > I used gdb to help me understand, however the condition
> >
> > 	if (fpextra&&  !IS_UPPER_REL(foreignrel))
> >
> > never evaluated to true with the query above.
> =

> Sorry, my explanation was not enough again, but I showed that query ("SE=
LECT
> a+b, random() FROM foreign_table GROUP BY a+b ORDER BY a+b;") to explain=
 why
> the following code bit is needed:
> =

> +       /*
> +        * If this includes an UPPERREL_ORDERED step, the given target, =
which
> +        * would be the final target to be applied to the resulting path=
,
> might
> +        * have different expressions from the underlying relation's rel=
target
> +        * (see make_sort_input_target()); adjust tlist eval costs.
> +        */
> +       if (fpextra&&  fpextra->target !=3D foreignrel->reltarget)
> +       {
> +               QualCost        oldcost =3D foreignrel->reltarget->cost;
> +               QualCost        newcost =3D fpextra->target->cost;
> +
> +               startup_cost +=3D newcost.startup - oldcost.startup;
> +               total_cost +=3D newcost.startup - oldcost.startup;
> +               total_cost +=3D (newcost.per_tuple - oldcost.per_tuple) =
* rows;
> +       }

Maybe I undestand now. Do the expressions (newcost.* - oldcost.*) reflect =
the
fact that, for the query

	SELECT a+b, random() FROM foreign_table GROUP BY a+b ORDER BY a+b;

the UPPERREL_ORDERED stage only needs to evaluate the random() function
because (a + b) was already evaluated during the UPPERREL_GROUP_AGG stage?

-- =

Antonin Houska
https://www.cybertec-postgresql.com