From: Richard Huxton <dev@archonet.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: Return relation table data in a single value CSV
Date: Wed, 18 Feb 2004 09:29:42 +0000
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On Tuesday 17 February 2004 23:33, Tom Lane wrote:
> Richard Huxton <dev@archonet.com> writes:
> > 2. Write a custom aggregate function (like sum()) to do the
> > concatenation. This is easy to do, but the order your ABC get processed
> > in is undefined.
>
> Actually, as of 7.4 it is possible to control the order of inputs to a
> custom aggregate.  You do something like this:
>
> 	SELECT foo, myagg(bar) FROM
> 	  (SELECT foo, bar FROM table ORDER BY foo, baz) AS ss
> 	GROUP BY foo
>
> The inner sub-select must order by the columns that the outer will group
> on; it can then order by additional columns that determine the sort
> order within each group.  Here, myagg() will see its input ordered by
> increasing values of baz.

Hmm - good to know, but I'm always wary of doing this sort of thing. It's 
exactly the sort of trick I look at 18 months later, fail to read my own 
comments and "tidy" it.

-- 
  Richard Huxton
  Archonet Ltd