From: "Jim Buttafuoco" <jim@contactbda.com>
To: Jan Dittmer <jdi@l4x.org>, pgsql-performance@postgresql.org
Reply-To: jim@contactbda.com
Subject: Re: Better way to write aggregates?
Date: Fri, 21 Apr 2006 08:27:32 -0400
Message-Id: <20060421122617.M81850@contactbda.com>
In-Reply-To: <444899B6.6090001@l4x.org>
References: <444899B6.6090001@l4x.org>
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Jan,

I write queries like this

CREATE VIEW parent_childs AS
SELECT
 	c.parent,
 	count(c.state) as childtotal,
 	sum(case when c.state = 1 then 1 else 0 end) as childstate1,
 	sum(case when c.state = 2 then 1 else 0 end) as childstate2,
 	sum(case when c.state = 3 then 1 else 0 end) as childstate3
 FROM child c
 GROUP BY parent;

---------- Original Message -----------
From: Jan Dittmer <jdi@l4x.org>
To: pgsql-performance@postgresql.org
Sent: Fri, 21 Apr 2006 10:37:10 +0200
Subject: [PERFORM] Better way to write aggregates?

> 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
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 6: explain analyze is your friend
------- End of Original Message -------