Re: DRAFT: Pass sk_attno to consistent function

=?utf-8?Q?Micha=C5=82_K=C5=82eczek?= <michal(at)kleczek(dot)org> writes:
> I understand extensibility of GIST makes many operators opaque to the planner and it is the “consistent” function that can perform optimisations (or we can come up with some additional mechanism during planning phase).
> Providing more information to “consistent” function would make it possible to implement optimisations not only for array scan keys but for ranges and others.

> What we can do is to provide the index attribute number (reduntantly) as option - it is going to work but is somewhat ugly - especially that this information is already available when calling consistent function.

While the proposed change is simple enough and wouldn't break
anything, I share Matthias' distaste for it, because the motivation
you've given for it is deeply unsatisfying and indeed troubling.
A GIST consistent function is surely the wrong place to be optimizing
away unnecessary partitions: that consideration is not unique to
GIST indexes (or even to indexscans), much less to particular opclasses.
Moreover, having a consistent function inspect catalog state seems
like a kluge of the first order, not least because there'd be no good
place to cache the lookup results, so you'd be doing them over and
over again.

I like Matthias' suggestion of seeing whether you could use a
planner support function to split up the query by partitions
during planning.

There are bits of what you mentioned that I would gladly take
patches for --- for example, I think the reason GIST lacks SAOP
support is mostly lack of round tuits, not that it'd be a bad
thing. But it's not clear to me that that alone would help you
much. The whole design as you've described it seems like multiple
layers of kluges, so that getting rid of any one of them doesn't
really make it not klugy. (I also have serious doubts about how well
it would perform for you, even with this additional kluge in place.
I don't think that a multidimensional GIST over zillions of rows will
perform very well: the upper tree layers are just not going to be very
selective.)

regards, tom lane