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To: =?iso-8859-1?Q?Domoszlai_L=E1szl=F3?= <dlacko@finit.hu>
Cc: "SQL PostgreSQL (E-mail)" <pgsql-sql@postgresql.org>
Subject: Re: set difference 
In-reply-to: <D6CD29CE4DF67F47A11FB6CF5EBE15D10ECC55@gedeon.finit.local> 
References: <D6CD29CE4DF67F47A11FB6CF5EBE15D10ECC55@gedeon.finit.local>
Comments: In-reply-to =?iso-8859-1?Q?Domoszlai_L=E1szl=F3?= <dlacko@finit.hu>
	message dated "Tue, 15 Oct 2002 14:52:23 +0200"
Date: Tue, 15 Oct 2002 10:25:39 -0400
Message-ID: <14808.1034691939@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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=?iso-8859-1?Q?Domoszlai_L=E1szl=F3?= <dlacko@finit.hu> writes:
> I would like to make symmetrical(set) difference in a query.
> But the simpliest way I could find is 

> select id from a 
> except
> select id from b
> union 
> select id from b
> except
> select id from a

> Is there any better solution for this problem?

One thing you should definitely do is change "union" to "union all".
"union" implies a pass of duplicate removal, which shouldn't be
necessary here (unless a or b individually contain duplicates and
you want to get rid of those too).

Another thing to try is
	(a union b) except (a intersect b)
(Again, you might be able to say union all instead of union.)
Not sure which will be faster.

			regards, tom lane