Re: Reduce "Var IS [NOT] NULL" quals during constant folding

On Tue, Apr 1, 2025 at 2:34 AM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> However, I gave up this idea because I realized it would require
> retrieving a whole bundle of catalog information that isn't needed
> until after the RelOptInfos are built, such as max_attr, pages,
> tuples, reltablespace, parallel_workers, extended statistics, etc.

Why is that bad? I mean, if we're going to need that information
anyway, then gathering it at earlier stage doesn't hurt. Of course, if
we move it too early, say before partition pruning, then we might
gather information we don't really need and hurt performance. But
otherwise it doesn't seem to hurt anything.

> And we may also need to create the IndexOptInfos for the relation's
> indexes. It seems to me that it's not a trivial task to move
> get_relation_info() before building the RelOptInfos, and more
> importantly, it's unnecessary most of the time.

But again, if the work is going to have to be done anyway, who cares?

> It could be argued that the separation of catalog information
> collection isn't very great, but it seems this isn't something new in
> this patch. So I respectfully disagree with your statement that "all
> the information that the planner needs about a particular relation is
> retrieved by get_relation_info()", and that "there's now just exactly
> this 1 thing that is done in a different place". For instance,
> relhassubclass and attgenerated are retrieved before expression
> preprocessing, a relation's constraint expressions are retrieved when
> setting the relation's size estimates, and more.

Nonetheless I think we ought to be trying to consolidate more things
into get_relation_info(), not disperse some of the things that are
there to other places.

--
Robert Haas
EDB: http://www.enterprisedb.com