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

Sergey Burladyan <eshkinkot@gmail.com> writes:

> # 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

While running this EXPLAIN backend use disk for a long time:
 TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND
                                                                     
21638 be/4 postgres    2.10 M/s    9.45 M/s  0.00 % 69.04 % postgres: postgres xxxxx xxx.xxx.xxx.xxx(50987) EXPLAIN

Why it read and write to disk 10 megabytes per second for EXPLAIN query? Cannot understand what is going on here :(