Hi After reading through the relevant parts of sytnax.sgml, create_aggregate.smgl and xaggr.sgml, I think I understand how these work - they work exactly like regular aggregates, except that some arguments are evaluated only once and passed to the final function instead of the transition function. The whole "ORDER BY" thing is just crazy syntax the standard mandates - a saner alternative would have been
ordered_set_agg(direct1,...,directN, WITHIN(arg1,...,argM)) or something like that, right? So whether "ORDER BY" implies any actual ordering is up to the ordered-set aggregate's final function. Or at least that's what xaggr.sgml seems to say Unlike the case for normal aggregates, the sorting of input rows for an ordered-set aggregate is <emphasis>not</> done behind the scenes, but is the responsibility of the aggregate's support functions. but that seems to contradict syntax.sgml which says The expressions in the <replaceable>order_by_clause</replaceable> are evaluated once per input row just like normal aggregate arguments, sorted as per the <replaceable>order_by_clause</replaceable>'s requirements, and fed to the aggregate function as input arguments. Also, xaggr.sgml has the following to explain why the NULLs are passed for all aggregated arguments to the final function, instead of simply not passing them at all While the null values seem useless at first sight, they are important because they make it possible to include the data types of the aggregated input(s) in the final function's signature, which may be necessary to resolve the output type of a polymorphic aggregate. Why do ordered-set aggregates required that, when plain aggregates are fine without it? array_agg(), for example, also has a result type that is determined by the argument type, yet it's final function doesn't take an argument of type anyelement, even though it returns anyarray. best regards, Florian Pflug -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
