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Re: string_agg delimiter having no effect with order by

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Thom Brown <thom(at)linux(dot)com>, pgsql-bugs <pgsql-bugs(at)postgresql(dot)org>
Subject: Re: string_agg delimiter having no effect with order by
Date: 2010-08-04 16:40:09
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Lists: pgsql-bugspgsql-hackers
On Wed, Aug 4, 2010 at 11:29 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> Oh, yeah.  I guess you need this:
>> select thing, string_agg(stuff, ',' order by stuff) from agg_test
>> group by thing;
>> Rather than this:
>> select thing, string_agg(stuff order by stuff, ',') from agg_test
>> group by thing;
>> It's all kinds of not obvious to me what the second one is supposed to
>> mean, but I remember this was discussed before.  Perhaps we need a
>> <note> somewhere about multi-argument aggregates.
> Done:
> +    <para>
> +     When dealing with multiple-argument aggregate functions, note that the
> +     <literal>ORDER BY</> clause goes after all the aggregate arguments.
> +     For example, this:
> + <programlisting>
> + SELECT string_agg(a, ',' ORDER BY a) FROM table;
> + </programlisting>
> +     not this:
> + <programlisting>
> + SELECT string_agg(a ORDER BY a, ',') FROM table;  -- not what you want
> + </programlisting>
> +     The latter syntax will be accepted, but <literal>','</> will be
> +     treated as a (useless) sort key.
> +    </para>

Oh, right, that's what it's supposed to mean.  Thanks for adding this.
 I suppose this confusion is only possible because string_agg has both
a one-argument and a two-argument form.

Robert Haas
The Enterprise Postgres Company

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