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

Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> I am thinking about implementation of median function. This function
> should be implemented in two ways:

> a) direct entering an ORDER BY clause for median funcall in gram.y
> b) general support for "preordered aggregates".

> I prefer plan b, because there are more similar aggregates - like
> Quantiles.

This seems like a great deal of mechanism to solve a very localized
problem.

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.

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.

regards, tom lane