Re: generic plans and "initial" pruning

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> That's not quite my question, though. Why do we ever build a non-flat
> range table in the first place? Like, instead of assigning indexes
> relative to the current subquery level, why not just assign them
> relative to the whole query from the start?

We could probably make that work, but I'm skeptical that it would
really be an improvement overall, for a couple of reasons.

(1) The need for merge-rangetables-and-renumber-Vars logic doesn't
go away. It just moves from setrefs.c to the rewriter, which would
have to do it when expanding views. This would be a net loss
performance-wise, I think, because setrefs.c can do it as part of a
parsetree scan that it has to perform anyway for other housekeeping
reasons; but the rewriter would need a brand new pass over the tree.
Admittedly that pass would only happen for view replacement, but
it's still not open-and-shut that there'd be a performance win.

(2) The need for varlevelsup and similar fields doesn't go away,
I think, because we need those for semantic purposes such as
discovering the query level that aggregates are associated with.
That means that subquery flattening still has to make a pass over
the tree to touch every Var's varlevelsup; so not having to adjust
varno at the same time would save little.

I'm not sure whether I think it's a net plus or net minus that
varno would become effectively independent of varlevelsup.
It'd be different from the way we think of them now, for sure,
and I think it'd take awhile to flush out bugs arising from such
a redefinition.

> I don't really expect that we're ever going to change this -- and
> certainly not on this thread. The idea of running around and replacing
> RT indexes all over the tree is deeply embedded in the system. But are
> we really sure we want to add a second kind of index that we have to
> run around and adjust at the same time?

You probably want to avert your eyes from [1], then ;-). Although
I'm far from convinced that the cross-list index fields currently
proposed there are actually necessary; the cost to adjust them
during rangetable merging could outweigh any benefit.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/CA+HiwqGjJDmUhDSfv-U2qhKJjt9ST7Xh9JXC_irsAQ1TAUsJYg(at)mail(dot)gmail(dot)com