To: Michael Fuhr <mike@fuhr.org>
cc: Steven Murdoch <psql+Steven.Murdoch@cl.cam.ac.uk>,
 pgsql-sql@postgresql.org
Subject: Re: Sorting items in aggregate function 
In-reply-to: <20060912181759.GA66966@winnie.fuhr.org> 
References: <20060912154628.GB4991@cl.cam.ac.uk>
	<20060912181759.GA66966@winnie.fuhr.org>
Comments: In-reply-to Michael Fuhr <mike@fuhr.org>
	message dated "Tue, 12 Sep 2006 12:17:59 -0600"
Date: Tue, 12 Sep 2006 16:37:55 -0400
Message-ID: <21365.1158093475@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Michael Fuhr <mike@fuhr.org> writes:
> On Tue, Sep 12, 2006 at 04:46:28PM +0100, Steven Murdoch wrote:
>> Here is an example of a setup and what I could like to achieve. Does
>> anyone have suggestions on what is the best way to get the desired
>> result?

> Use the aggregate over an ordered subquery:

> SELECT name, trim(concat(code || ' ')) AS codes
> FROM (
>   SELECT a.name, b.code
>   FROM a
>   LEFT JOIN ab ON a.id = ab.a_id
>   LEFT JOIN b ON ab.b_id = b.id
>   ORDER BY b.code
> ) AS s
> GROUP BY name
> ORDER BY name;

Note that if you need to GROUP in the outer query, it's best to sort the
inner query's output first by the outer query's grouping:

SELECT name, trim(concat(code || ' ')) AS codes
FROM (
  SELECT a.name, b.code
  FROM a
  LEFT JOIN ab ON a.id = ab.a_id
  LEFT JOIN b ON ab.b_id = b.id
  ORDER BY a.name, b.code
           ^^^^^^^^^^^^^^
) AS s
GROUP BY name
ORDER BY name;

This way will still work if the planner decides to use a GroupAggregate
(which in fact it probably will, if it sees it can avoid another sort
step).  The way Michael showed will only work if the plan uses
HashAggregate --- if the planner decides it needs Sort+GroupAggregate
in the outer query, the re-sort will probably destroy the ordering
by b.code.

			regards, tom lane