From: Heikki Linnakangas <heikki@enterprisedb.com>
To: Stefano Dal Pra <s.dalpra@gmail.com>
Cc: pgsql-sql@postgresql.org
Cc: pgsql-performance@postgresql.org
Subject: Re: two queryes in a single tablescan
Date: Wed, 17 Oct 2007 14:00:58 +0100
Message-ID: <4716078A.1030505@enterprisedb.com> (raw)
In-Reply-To: <290a5abc0710170530v215cb2b0q4fa000ad43096a28@mail.gmail.com>
References: <290a5abc0710170530v215cb2b0q4fa000ad43096a28@mail.gmail.com>
Stefano Dal Pra wrote:
> suppose you have a large table tab and two (or more) queryes like this:
>
> SELECT count(*),A FROM tab WHERE C GROUP BY A;
> SELECT count(*),B FROM tab WHERE C GROUP BY B;
>
> is there any way to get both results in a single query,
> eventually through stored procedure?
> The retrieved [count(*),A] ; [count(*),B)] data couldnt fit
> on a single table, of course.
>
> The main goal would be to get multiple results while scanning the
> table[s] once only
> thus getting results in a faster way.
>
> This seems to me quite a common situation but i have no clue whether a neat
> solution can be implemented through stored procedure.
With a temp table:
CREATE TEMPORARY TABLE tmp AS SELECT COUNT(*) as rows, a,b FROM WHERE C
GROUP BY a,b;
SELECT SUM(rows), a FROM tmp GROUP BY a;
SELECT SUM(rows), b FROM tmp GROUP BY b;
DROP TABLE tmp;
(Using temp tables in plpgsql procedures doesn't quite work until 8.3.
But you can use dynamic EXECUTE as a work-around. There used to be a FAQ
entry about that, but apparently it's been removed because the problem
has been fixed in the upcoming release.)
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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To: pgsql-sql@postgresql.org
Cc: heikki@enterprisedb.com, s.dalpra@gmail.com, pgsql-performance@postgresql.org
Subject: Re: two queryes in a single tablescan
In-Reply-To: <4716078A.1030505@enterprisedb.com>
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