From: Tom Lane <tgl@sss.pgh.pa.us>
To: josh@agliodbs.com
Cc: Lorraine.Dewey@companiongroup.com
Cc: pgsql-sql@postgresql.org
Subject: Re: Optomizing left outer joins
Date: Wed, 23 Apr 2003 22:37:38 -0400
Message-ID: <10221.1051151858@sss.pgh.pa.us> (raw)
In-Reply-To: <200304231149.32514.josh@agliodbs.com>
References: <ebfd27c8.0304210714.66e3ae75@posting.google.com>
<200304231149.32514.josh@agliodbs.com>
Josh Berkus <josh@agliodbs.com> writes:
> FWIW, outer joins are slower than regular joins on all RDBMSs I've tested --
> often up to 5 times slower. I'm not sure whether this is just the planner
> restricitons inherent in an outer join, or whether this is something about
> the required join algorithm itself.
There isn't anything fundamentally slower about left join vs plain join
(unless, perhaps, the left join has to generate a really large number
of null-extended rows that the plain join would not generate). My money
is on the left join having prevented some optimization that could be
applied in the plain-join case. But since we have not been shown any
exact details of the tables or the query, it's impossible to do more
than wager...
regards, tom lane
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, josh@agliodbs.com, Lorraine.Dewey@companiongroup.com
Subject: Re: Optomizing left outer joins
In-Reply-To: <10221.1051151858@sss.pgh.pa.us>
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