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From: Gunnar \"Nick\" Bluth <gunnar.bluth.extern@elster.de>
To: pgsql-performance@postgresql.org
Subject: Re: Estimation row error
Date: Tue, 15 Dec 2015 10:25:30 +0100
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Am 15.12.2015 um 09:05 schrieb Mathieu VINCENT:
> Hello,
> 
> No one to help me to understand this bad estimation rows ?

Well,

on a rather beefy machine, I'm getting quite a different plan:
http://explain.depesz.com/s/3y5r

Which may be related to this setting:
perftest=# show default_statistics_target ;
 default_statistics_target
---------------------------
 1000
(1 Zeile)


I guess the wrong row assumption (which I get as well!) is caused by the
given correlation of t3.c1 and t3.c2 (which the planner doesn't "see").

Tomas Vondra has written a nice blog post, covering that topic as well:
http://blog.pgaddict.com/posts/common-issues-with-planner-statistics

AFAIK, 9.5 has received some improvements in that field, but I didn't
try that yet.

Best regards,

Nick

> 
> Mathieu VINCENT
> 
> 2015-12-11 12:35 GMT+01:00 Mathieu VINCENT
> <mathieu.vincent@pmsipilot.com <mailto:mathieu.vincent@pmsipilot.com>>:
> 
>     Sorry, I forget to precise Postgresql version
> 
>     'PostgreSQL 9.4.4 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
>     4.4.7 20120313 (Red Hat 4.4.7-11), 64-bit'
> 
> 
>     BR
> 
>     Mathieu VINCENT
> 
>     	
> 
> 
>     2015-12-11 9:53 GMT+01:00 Mathieu VINCENT
>     <mathieu.vincent@pmsipilot.com <mailto:mathieu.vincent@pmsipilot.com>>:
> 
>         Hello,
> 
>         I would like to know how row estimation is calculed by explain ?
>         In my execution plan, this estimation is extremely wrong (267
>         instead of 198000)
>         I reproduced this estimation error in this simple case :
> 
>         drop table if exists t1;
>         drop table if exists t2;
>         drop table if exists t3;
>         drop table if exists t4;
> 
>         create table t1 as select generate_Series(1,300000) as c1; 
>         create table t2 as select generate_Series(1,400) as c1; 
>         create table t3 as select generate_Series(1,200000)%100 as
>         c1,generate_Series(1,200000) as c2;
>         create table t4 as select generate_Series(1,200000) as c1;
> 
>         alter table t1 add PRIMARY KEY (c1);
>         alter table t2 add PRIMARY KEY (c1);
>         alter table t3 add PRIMARY KEY (c1,c2);
>         create index on t3 (c1);
>         create index on t3 (c2);
>         alter table t4 add PRIMARY KEY (c1);
> 
>         analyze t1;
>         analyze t2;
>         analyze t3;
>         analyze t4;
> 
>         EXPLAIN (analyze on, buffers on, verbose on)
>         select 
>         *
>         from 
>         t1 t1
>         inner join t2 on t1.c1=t2.c1
>         inner join t3 on t2.c1=t3.c1
>         inner join t4 on t3.c2=t4.c1
> 
>         Explain plan :
>         http://explain.depesz.com/s/wZ3v
> 
>         I think this error may be problematic because planner will
>         choose nested loop instead of hash joins for ultimate join. Can
>         you help me to improve this row estimation ? 
> 
>         Thank you for answering
> 
>         Best Regards,
>         <http://www.psih.fr/;	PSIH Décisionnel en santé
>         Mathieu VINCENT 
>         Data Analyst
>         PMSIpilot - 61 rue Sully - 69006 Lyon - France
> 
> 
> 


-- 
Gunnar "Nick" Bluth
DBA ELSTER

Tel:   +49 911/991-4665
Mobil: +49 172/8853339


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