Re: Proposal: Pre ordered aggregates, default ORDER BY clause for aggregates - median support - Mailing list pgsql-hackers

2009/12/20 Andrew Gierth <andrew@tao11.riddles.org.uk>:
>>>>>> "Pavel" == Pavel Stehule <pavel.stehule@gmail.com> writes:
>
>  > 2009/12/20 Tom Lane <tgl@sss.pgh.pa.us>:
>  >> 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.
>
> I absolutely agree with Tom here and for some quite specific reasons.
>
> An optimal (or at least more optimal than is currently possible)
> implementation of median() on top of the ordered-agg code as it stands
> requires additions to the aggregate function interface: the median agg
> implementation would have to, as a minimum, know how many rows of
> sorted input are available. In addition, it would be desirable for it
> to have direct (and possibly bidirectional) access to the tuplesort.

I was thinking about some new kind of aggregates based on tuple-store.


>
> Now, if we look at how ordered aggs ought to be optimized, it's clear
> that the planner should take the ordering costs into account and
> consider plans that order the input instead. Once you do this, then
> there's no longer any pre-computed count or tuplesort object
> available, so if you'd implemented a better median() before the
> optimizations, you'd end up either having to forgo the optimization or
> break the median() agg again; clearly not something we want.
>

Information about count of rows are not detected in planner time. Thishave to by available in any executor's sort node.
Sothis value will 
be available every time - index scan is problem. Interesting is Greg's
info about O(N) algorithms. Then we can implement median as classic
aggregate.

> Plus, a feature that I intentionally omitted from the ordered-aggs
> patch is the ability to do ordered-aggs as window functions. This is
> in the spec, but (a) there were conflicting patches for window
> functions on the table and (b) in my opinion, much of the work needed
> to implement ordered-agg-as-window-func in an effective manner is
> dependent on doing more work on optimization first (or at least will
> potentially become easier as a result of that work).
>

I fully agree, so agg(x order by x) needs some work more - so general
design is premature.

On second hand - any internal implementation of median will be faster,
then currently used techniques.

Pavel

> So I think both the optimization issue and the window functions issue
> would be best addressed before trying to build any additional features
> on top of what we have so far.
>
> --
> Andrew (irc:RhodiumToad)
>