From: Domoszlai László <dlacko@finit.hu>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: SQL PostgreSQL (E-mail) <pgsql-sql@postgresql.org>
Subject: Re: set difference
Date: Wed, 16 Oct 2002 15:44:42 +0200
Message-ID: <D6CD29CE4DF67F47A11FB6CF5EBE15D1375753@gedeon.finit.local> (raw)
Tom Lane wrote:
> > 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.
It's likely faster but my problem is I have to run queries twice. I've looked at
setOp executor and it seems symmetrical differencial can run as time as an EXCEPT.
I would write it, because it speeds up my query very much, but I don't know if it is worth. So, do you think a DIFFERENCE or EXCEPT SYMMETRIC clause will be reasonable
feauture for postgresql?
Laca
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To: pgsql-sql@postgresql.org
Cc: dlacko@finit.hu, tgl@sss.pgh.pa.us
Subject: Re: set difference
In-Reply-To: <D6CD29CE4DF67F47A11FB6CF5EBE15D1375753@gedeon.finit.local>
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