Message-ID: <444899B6.6090001@l4x.org>
Date: Fri, 21 Apr 2006 10:37:10 +0200
From: Jan Dittmer <jdi@l4x.org>
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To: pgsql-performance@postgresql.org
Subject: Better way to write aggregates?
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Hi,

I more or less often come about the problem of aggregating a
child table counting it's different states. The cleanest solution
I've come up with so far is:

BEGIN;
CREATE TABLE parent (
	id int not null,
  	name text not null,
	UNIQUE(id)
);

CREATE TABLE child (
	name text not null,
	state int not null,
	parent int not null references parent(id)
);

CREATE VIEW parent_childs AS
SELECT
	c.parent,
	count(c.state) as childtotal,
	count(c.state) - count(nullif(c.state,1)) as childstate1,
	count(c.state) - count(nullif(c.state,2)) as childstate2,
	count(c.state) - count(nullif(c.state,3)) as childstate3
FROM child c
GROUP BY parent;

CREATE VIEW parent_view AS
SELECT p.*,
pc.*
FROM parent p
LEFT JOIN parent_childs pc ON (p.id = pc.parent);
COMMIT;

Is this the fastest way to build these aggregates (not considering
tricks with triggers, etc)? The count(state) - count(nullif(...)) looks
a bit clumsy.
I also experimented with a pgsql function to sum these up, but considered
it as not-so-nice and it also always forces a sequential scan on the
data.

Thanks for any advice,

Jan