From: Tom Lane <tgl@sss.pgh.pa.us>
To: azajac@ang.com.pl
Cc: pgsql-performance@postgreSQL.org
Subject: Re: Problem with mergejoin performance
Date: Mon, 13 Sep 2010 11:16:40 -0400
Message-ID: <12711.1284391000@sss.pgh.pa.us> (raw)
In-Reply-To: <005301cb5354$f0797100$d16c5300$@com.pl>
References: <000001cb5307$78955480$69bffd80$@com.pl>
<11777.1284387988@sss.pgh.pa.us>
<005301cb5354$f0797100$d16c5300$@com.pl>
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<azajac@ang.com.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
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, azajac@ang.com.pl
Subject: Re: Problem with mergejoin performance
In-Reply-To: <12711.1284391000@sss.pgh.pa.us>
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