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Re: planner's midjudge number of rows resulting, despite pretty obvious join

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: Grzegorz Jaśkiewicz <gryzman(at)gmail(dot)com>
Cc: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: planner's midjudge number of rows resulting, despite pretty obvious join
Date: 2009-02-24 14:43:01
Message-ID: 162867790902240643p539b0f05rf7477cdde07b7459@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
Hello

2009/2/24 Grzegorz Jaśkiewicz <gryzman(at)gmail(dot)com>:
> explain select ss, ARRAY(select id from foo where ss>0 and id between
> 7 and 156 order by random() limit 3) as v from
> generate_series(1,1000000) ss;
>                                     QUERY PLAN
> ------------------------------------------------------------------------------------
>  Function Scan on generate_series ss  (cost=0.00..9381.22 rows=1000 width=4)
>   SubPlan
>     ->  Limit  (cost=9.36..9.37 rows=3 width=8)
>           ->  Sort  (cost=9.36..9.74 rows=150 width=8)
>                 Sort Key: (random())
>                 ->  Result  (cost=0.00..7.42 rows=150 width=8)
>                       One-Time Filter: ($0 > 0)
>                       ->  Seq Scan on foo  (cost=0.00..7.05 rows=150 width=8)
>                             Filter: ((id >= 7) AND (id <= 156))
> (9 rows)
>
> :(
>
> no matter if I change last generate_series's range, it will always
> estimate 1000 rows...
>
>

There are not dynamic estimator for SRF function. You can change it
statically via ROWS flag - default is 1000 rows.

postgres=# create or replace function fooo() returns setof int as $$
select * from (values(10),(20)) x$$ language sql;
CREATE FUNCTION
postgres=# explain select * from fooo();
                          QUERY PLAN
--------------------------------------------------------------
 Function Scan on fooo  (cost=0.00..260.00 rows=1000 width=4)
(1 row)

postgres=# create or replace function fooo() returns setof int as $$
select * from (values(10),(20)) x$$ language sql rows 432;
CREATE FUNCTION
postgres=# explain select * from fooo();
                         QUERY PLAN
-------------------------------------------------------------
 Function Scan on fooo  (cost=0.00..112.32 rows=432 width=4)
(1 row)

postgres=#

regards
Pavel Stehule

> --
> GJ
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Subject: planner's midjudge number of rows resulting, despite pretty obvious join

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