Skip site navigation (1) Skip section navigation (2)

Re: limit clause breaks query planner?

From: "Pavel Stehule" <pavel(dot)stehule(at)gmail(dot)com>
To: "Guillaume Cottenceau" <gc(at)mnc(dot)ch>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: limit clause breaks query planner?
Date: 2008-09-02 07:46:58
Message-ID: 162867790809020046ne597803qa7f597a4fbd54d45@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
2008/9/2 Guillaume Cottenceau <gc(at)mnc(dot)ch>:
> "Pavel Stehule" <pavel.stehule 'at' gmail.com> writes:
>
>> Hello
>>
>> 2008/9/1 David West <david(dot)west(at)cusppoint(dot)com>:
>>> Thanks for your suggestion but the result is the same.
>>>
>>> Here is the explain analyse output from different queries.
>>> Select * from my_table where A is null and B = '21' limit 15
>>>
>>> "Limit  (cost=0.00..3.68 rows=15 width=128) (actual time=85837.043..85896.140 rows=15 loops=1)"
>>> "  ->  Seq Scan on my_table this_  (cost=0.00..258789.88 rows=1055580 width=128) (actual time=85837.038..85896.091 rows=15 loops=1)"
>>> "        Filter: ((A IS NULL) AND ((B)::text = '21'::text))"
>>> "Total runtime: 85896.214 ms"
>>>
>>
>> I see it - problem is in statistics - system expect 1055580, but there
>> is only 15 values.
>
> Aren't you rather seeing the effect of the limit clause?

yes, true, my mistake

Pavel

>
> gc=# create table foo ( bar int );
> CREATE TABLE
> gc=# insert into foo ( select generate_series(0, 10000000) / 1000000 );
> INSERT 0 10000001
> gc=# analyze foo;
> ANALYZE
> gc=# explain analyze select * from foo where bar = 8 limit 15;
>                                                     QUERY PLAN
> ---------------------------------------------------------------------------------------------------------------------
>  Limit  (cost=0.00..2.30 rows=15 width=4) (actual time=2379.878..2379.921 rows=15 loops=1)
>   ->  Seq Scan on foo  (cost=0.00..164217.00 rows=1070009 width=4) (actual time=2379.873..2379.888 rows=15 loops=1)
>         Filter: (bar = 8)
>  Total runtime: 2379.974 ms
>
> (on 8.3.1)
>
> --
> Guillaume Cottenceau, MNC Mobile News Channel SA, an Alcatel-Lucent Company
> Av. de la Gare 10, 1003 Lausanne, Switzerland - direct +41 21 317 50 36
>

In response to

pgsql-performance by date

Next:From: Greg SmithDate: 2008-09-02 07:57:27
Subject: Re: slow update of index during insert/copy
Previous:From: Guillaume CottenceauDate: 2008-09-02 07:44:33
Subject: Re: limit clause breaks query planner?

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group