Re: Problems with plan estimates in postgres_fdw

From: Antonin Houska <ah(at)cybertec(dot)at>
To: Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Problems with plan estimates in postgres_fdw
Date: 2019-02-18 14:21:11
Message-ID: 30524.1550499671@localhost
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Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:

> (2019/02/15 21:46), Antonin Houska wrote:
> > ok, I understand now. I assume that the patch
> >
> > https://www.postgresql.org/message-id/5C66A056.60007%40lab.ntt.co.jp
> >
> > obsoletes the code snippet we discussed above.
>
> Sorry, I don't understand this. Could you elaborate on that?

I thought that the assignments

+ startup_cost += outerrel->reltarget->cost.startup;

and

+ run_cost += outerrel->reltarget->cost.per_tuple * input_rows;

in the patch you posted in
https://www.postgresql.org/message-id/5C66A056.60007%40lab.ntt.co.jp do
replace

+ startup_cost += foreignrel->reltarget->cost.startup;

and

+ run_cost += foreignrel->reltarget->cost.per_tuple * rows;

respectively, which you originally included in the following part of
0001-postgres_fdw-Perform-UPPERREL_ORDERED-step-remotely-v3.patch:

@@ -2829,6 +2872,22 @@ estimate_path_cost_size(PlannerInfo *root,
run_cost += foreignrel->reltarget->cost.per_tuple * rows;
}

+ /*
+ * If this is an UPPERREL_ORDERED step on the final scan/join
+ * relation, the costs obtained from the cache wouldn't yet contain
+ * the eval cost for the final scan/join target, which would have been
+ * updated by apply_scanjoin_target_to_paths(); add the eval cost now.
+ */
+ if (fpextra && !IS_UPPER_REL(foreignrel))
+ {
+ /* The costs should have been obtained from the cache. */
+ Assert(fpinfo->rel_startup_cost >= 0 &&
+ fpinfo->rel_total_cost >= 0);
+
+ startup_cost += foreignrel->reltarget->cost.startup;
+ run_cost += foreignrel->reltarget->cost.per_tuple * rows;
+ }
+
/*
* Without remote estimates, we have no real way to estimate the cost
* of generating sorted output. It could be free if the query plan

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26, A-2700 Wiener Neustadt
Web: https://www.cybertec-postgresql.com

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