Re: Estimation row error

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(dot)vincent(at)pmsipilot(dot)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
>