Aggregate ORDER BY patch
in [PgSql]
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From: Pavel Stehule on 20 Dec 2009 10:15 2009/12/19 Marko Tiikkaja <marko.tiikkaja(a)cs.helsinki.fi>: > On 2009-12-15 23:10 +0200, Tom Lane wrote: >> >> Andrew Gierth<andrew(a)tao11.riddles.org.uk> Â writes: >>> >>> Notice that there are cases where agg(distinct x order by x) is >>> nondeterministic while agg(distinct x order by x,y) is deterministic. >> >> Well, I think what you're really describing is a case where you're using >> the wrong sort opclass. Â If the aggregate can distinguish two values of >> x, and the sort operator can't, use another sort operator that can. >> >> If we really wanted to take the above seriously, my opinion is that >> we ought to introduce DISTINCT ON in aggregates. Â However, at that >> point you lose the argument of standard syntax, so it's not real >> clear why you shouldn't just fall back on >> Â Â Â Â select agg(x) from (select distinct on (x) x .... order by x,y) > > FWIW, in my opinion the idea behind this patch is to not fall back on hacks > like that. Â This patch already goes beyond the standard and having this > seems like a useful feature in some cases. Â Although the DISTINCT ON syntax > would have a bit more resemblance on the existing syntax, I'd still like to > see agg(distinct x order by x,y). > when we are talking about extensions - did you thing about LIMIT clause? select agg(distinct x order by x limit 10) .. Regards Pavel > Just my $0.02. > > > Regards, > Marko Tiikkaja > > -- > Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-hackers > -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers |