Re: Optimization issue of branching UNION ALL

On Thu, Dec 22, 2022 at 9:50 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Andrey Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru> writes:
> > Superficial study revealed possibly unnecessary operations that could be
> > avoided:
> > 1. Walking across a query by calling substitute_phv_relids() even if
> > lastPHId shows that no one phv is presented.
>
> Yeah, we could do that, and it'd help some.

I noticed we also check 'parse->hasSubLinks' when we fix PHVs and
AppendRelInfos in pull_up_simple_subquery. I'm not sure why we have
this check. It seems not necessary.

In remove_result_refs, I don't think we need to check 'lastPHId' again
before calling substitute_phv_relids, since it has been checked a few
lines earlier.

Thanks
Richard