From: Tom Lane <tgl@sss.pgh.pa.us>
To: Domoszlai László <dlacko@finit.hu>
Cc: SQL PostgreSQL (E-mail) <pgsql-sql@postgresql.org>
Subject: Re: set difference
Date: Tue, 15 Oct 2002 10:25:39 -0400
Message-ID: <14808.1034691939@sss.pgh.pa.us> (raw)
In-Reply-To: <D6CD29CE4DF67F47A11FB6CF5EBE15D10ECC55@gedeon.finit.local>
References: <D6CD29CE4DF67F47A11FB6CF5EBE15D10ECC55@gedeon.finit.local>
=?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
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, dlacko@finit.hu
Subject: Re: set difference
In-Reply-To: <14808.1034691939@sss.pgh.pa.us>
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