Re: Looks like merge join planning time is too big, 55 seconds

Le 01/08/2013 11:55, Sergey Burladyan a écrit :
> Hello, i have a problem with planning time, I do not understand why this
> can happen.
>
> PostgreSQL 9.2.2 on x86_64-unknown-linux-gnu, compiled by gcc-4.4.real
> (Debian 4.4.5-8) 4.4.5, 64-bit
>
> # explain
> # select i.item_id, u.user_id from items i
> # left join users u on u.user_id = i.user_id
> # where item_id = 169946840;
>                                           QUERY PLAN
>
> ----------------------------------------------------------------------------------------------
>  Nested Loop Left Join  (cost=0.00..397.14 rows=1 width=16)
>    ->  Index Scan using items_item_ux on items i  (cost=0.00..358.84
> rows=1 width=16)
>          Index Cond: (item_id = 169946840)
>    ->  Index Only Scan using users_user_id_pkey on users u
>  (cost=0.00..38.30 rows=1 width=8)
>          Index Cond: (user_id = i.user_id)
>
> time: 55919.910 ms
>
> # set enable_mergejoin to off;
>
> # explain
> select i.item_id, u.user_id from items i
> left join users u on u.user_id = i.user_id
> where item_id = 169946840;
>                                           QUERY PLAN
>
> ----------------------------------------------------------------------------------------------
>  Nested Loop Left Join  (cost=0.00..397.14 rows=1 width=16)
>    ->  Index Scan using items_item_ux on items i  (cost=0.00..358.84
> rows=1 width=16)
>          Index Cond: (item_id = 169946840)
>    ->  Index Only Scan using users_user_id_pkey on users u
>  (cost=0.00..38.30 rows=1 width=8)
>          Index Cond: (user_id = i.user_id)
>
> time: 28.874 ms
>
> --
> Sergey Burladyan

Hello,

If you leave enable_mergejoin to on, what happens if you run the explain
two time in a row ? Do you get the same planning time ?

At first look, this reminds me some catalog bloat issue. Can you provide
the result of these queries :
SELECT pg_size_pretty(pg_table_size('pg_class')) AS size_pg_class;
SELECT pg_size_pretty(pg_table_size('pg_attribute')) AS size_pg_attribute;

Thanks
--
Thomas Reiss
Consultant Dalibo
http://dalibo.com - http://dalibo.org