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Re: RE : RE : BUG #3519: Postgres takes the wrong query plan resulting in performance issues

From: Gregory Stark <stark(at)enterprisedb(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>, "Mouhamadou Dia" <MDia(at)accovia(dot)com>, <pgsql-bugs(at)postgresql(dot)org>
Subject: Re: RE : RE : BUG #3519: Postgres takes the wrong query plan resulting in performance issues
Date: 2007-08-07 08:40:22
Message-ID: 87myx3louh.fsf@oxford.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-bugs
"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Gregory Stark <stark(at)enterprisedb(dot)com> writes:
>> The structure of your query is a whole series of left outer joins, the result
>> of which is then (inner) joined with one more table. The outer joins return a
>> whole lot of records but the inner join is only going to match a few of them.
>
> Hmmm ... actually I see 6 tables inside the join-tree and four more
> loose in the FROM-clause, ten relations altogether.  Which means the OP
> is falling foul of from_collapse_limit, and it's not investigating every
> possible join order.  Try setting from_collapse_limit to more than 10.

The three other loose ones are attached to a table inside outer joins though.
Doesn't that prevent any possibility of them being done earlier? But the first
one looks like it ought to be driving the join.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com


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