Re: BRIN indexes for MAX, MIN, ORDER BY?

Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Gavin Wahl wrote:
>> It seems trivial to accelerate a MAX or MIN query with a BRIN index. You
>> just find the page range with the largest/smallest value, and then only
>> scan that one. Would that be hard to implement? I'm interested in working
>> on it if someone can give me some pointers.

> I think this means you need to represent this operation as a specific
> Path in some way.  See build_minmax_path() and its callers in planagg;
> you probably need to tweak preprocess_minmax_aggregates() to consider
> this.

> This doesn't look like a simple project to me, mind.

>> Somewhat harder but still possible would be using BRIN indexes to
>> accelerate ORDER BY. This would require a sorting algorithm that can take
>> advantage of mostly-sorted inputs. You would sort the page ranges by their
>> minimum or maximum value, then feed the sorting algorithm in that order.

> I wouldn't know where to start for this.  Maybe once Tom is done with
> planner rejiggering it would make sense to consider looking at how to do
> it.

Yeah.  I would urgently recommend that people *not* try to build new
things like planagg.c right now.  A large part of the point of upper
planner path-ification is to have a less grotty way of dealing with
things like specialized aggregate implementations.

(And yes, I am working on that.  Honest.)
        regards, tom lane