On Friday 13 November 2009 16:35:08 Tom Lane wrote: > Greg Stark <gsst...@mit.edu> writes: > > On Fri, Nov 13, 2009 at 7:54 AM, Heikki Linnakangas > > > > <heikki.linnakan...@enterprisedb.com> wrote: > >> Andrew Gierth wrote: > >>> Herewith a patch to implement agg(foo ORDER BY bar) with or without > >>> DISTINCT, etc. > >> > >> What does that mean? Aggregate functions are supposed to be commutative, > >> right? > > > > We certainly have non-commutative agggregates currently, notably > > array_agg() > > Right. The fact that none of the standard aggregates are > order-sensitive doesn't mean that it's not useful to have user-defined > ones that are. Currently we suggest fetching from an ordered sub-select > if you want to use an aggregate that is input order sensitive. This > patch just provides an alternative (and equally nonstandard) notation > for that. > > I'm not entirely convinced that adding ORDER BY here is a good idea, > partly because it goes so far beyond the spec and partly because it's > not going to be easily optimizable. But I can see that there is a > use-case. The spec supports the ORDER BY syntax for the xmlagg aggregate...
Andres -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers