Re: Problem with mergejoin performance

[ Please keep the list cc'd ]

<azajac(at)ang(dot)com(dot)pl> writes:
>> What exactly are those join key values, and what are the min/max values
>> in bug_t2?

> min of Bug_t1.id = 42,
> max of Bug_t1.id = 393065,
> min of Bug_t2.id = 352448,
> max of Bug_t2.id = 388715,

> select count(id) from bug_t2
> 29
> select count(*) from bug_t2
> 278417
> And because there is only 29 not null records in bug_t2:

Oh, that's the real problem: thousands of nulls in bug_t2.
The planner is thinking those don't have to be scanned, but
the executor wasn't on the same page until very recently:
http://archives.postgresql.org/pgsql-committers/2010-05/msg00334.php

That patch will be in the next 8.3 update.

regards, tom lane