Message-ID: <1118256395.42a73d0bca492@webmail.dmi.dk>
Date: Wed,  8 Jun 2005 20:46:35 +0200
From: Kim Bisgaard <kib+pg@dmi.dk>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: full outer performance problem 
References: <42A6BC64.4010001@dmi.dk> <1275.1118239415@sss.pgh.pa.us>
In-Reply-To: <1275.1118239415@sss.pgh.pa.us>
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Quoting Tom Lane <tgl@sss.pgh.pa.us>:

> Kim Bisgaard <kib+pg@dmi.dk> writes:
> > SELECT station_id, timeobs,temp_grass, temp_dry_at_2m
> >         FROM temp_dry_at_2m a
> >         FULL OUTER JOIN temp_grass b
> >         USING (station_id, timeobs)
> >         WHERE station_id = 52981
> >           AND timeobs = '2004-1-1 0:0:0'
>
> > explain analyse SELECT b.station_id, b.timeobs,temp_grass, temp_dry_at_2m
> >         FROM temp_dry_at_2m a
> >         FULL OUTER JOIN temp_grass b
> >         USING (station_id, timeobs)
> >         WHERE b.station_id = 52981
> >           AND b.timeobs = '2004-1-1 0:0:0'
>
> > Why will PostgreSQL not use the same plan for both these queries - they
> > are virtually identical??
>
> Because they're semantically completely different.  The second query is
> effectively a RIGHT JOIN, because join rows in which b is all-null will
> be thrown away by the WHERE.  The optimizer sees this (note your second
> plan doesn't use a Full Join step anywhere) and is able to produce a
> much better plan.  Full outer join is difficult to optimize, in part
> because we have no choice but to use a merge join for it --- the other
> join types don't support full join.
>
> 			regards, tom lane
>


Yes I am aware that they are not "identical", they also give different results,
but the data nessesary to compute the results is (0-2 rows, 0-1 row from each
table), and thus ideally have the potential to have similar performance - to my
head anyway, but I may not have grasped the complete picture yet :-)

Regards,
Kim.