Re: [PATCH] Negative Transition Aggregate Functions (WIP)

On Mar5, 2014, at 18:37 , Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Dean Rasheed <dean.a.rasheed@gmail.com> writes:
>> I think we really need a larger consensus on this though, so I'd be
>> interested to hear what others think.
> 
> My advice is to lose the EXPLAIN output entirely.  If the authors of
> the patch can't agree on what it means, what hope have everyday users
> got of making sense of it?

The question isn't what the current output means, but whether it's a
good metric to report or not.

If we don't report anything, then how would a user check whether a query
is slow because of O(n^2) behaviour of a windowed aggregate, or because
of some other reasons? If inevitability where a purely static property,
then maybe we could get away with that, and say "check whether your
aggregates are invertible or not". But since we have partially invertible
aggregates, the performance characteristics depends on the input data,
so we IMHO need some way for users to check what's actually happening.

best regards,
Florian Pflug