DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta;
	h=received:message-id:date:from:user-agent:mime-version:to:cc:subject:references:in-reply-to:content-type:content-transfer-encoding:sender;
	b=KDyB6P5bPY7YqsrIhrf/gRV4uGIJCuRa2C8AwPNYbxfm3pYx+TPFtlieXeb0ZuIMb60Ask4BDavsXEoOuYVQAFfyx5vAtCI5MAIipJxwTrc2/aLX2OS9H+e71vdTGn5iNd2QtdRrdK9cZGyu04k4+m1WiNLgr2Q1d1iyWkn1FKg=
Message-ID: <4716078A.1030505@enterprisedb.com>
Date: Wed, 17 Oct 2007 14:00:58 +0100
From: Heikki Linnakangas <heikki@enterprisedb.com>
User-Agent: Icedove 1.5.0.12 (X11/20070730)
MIME-Version: 1.0
To: Stefano Dal Pra <s.dalpra@gmail.com>
CC: pgsql-sql@postgresql.org,  pgsql-performance@postgresql.org
Subject: Re: two queryes in a single tablescan
References: <290a5abc0710170530v215cb2b0q4fa000ad43096a28@mail.gmail.com>
In-Reply-To: <290a5abc0710170530v215cb2b0q4fa000ad43096a28@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

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