Re: Proposal: Pre ordered aggregates, default ORDER BY clause for aggregates - median support

Hi from a real user :)

Le 20 déc. 2009 à 22:08, Tom Lane a écrit :
> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
>> b) general support for "preordered aggregates".
>
> I think that we've already expanded the capabilities of aggregates
> a great deal for 8.5, and we should let it sit as-is for a release
> or two and see what the real user demand is for additional features.

All we can have in PostgreSQL without needing to resort to either PLs or application code is worth it from here, and I can already picture the smiling on our developers face when I say them median() is there by default.

> I'm particularly concerned by the fact that the feature set is already
> far out in front of what the planner can optimize effectively (e.g.,
> there's no ability to combine the work when multiple aggregates need the
> same sorted data). The more features we add on speculation, the harder
> it's going to be to close that gap.
>
> Another risk is that features added now might preclude adding others
> later.

Now, I have no idea if augmenting the aggregate properties with an optional sorting step is the right approach, but it sounds right on spot (general enough without being over engineering). I guess it would give the planner the same information as if the user did type the extra order by himself, so I'm not sure how much your remarks would apply?

I mean we already have explicit user ordering in aggregates at call site, adding the exact same information in the aggregate definition itself surely isn't going to be such a change there?

Regards,
--
dim