Re: Check SubPlan clause for nonnullable rels/Vars

* I don't believe you can prove anything from an ALL_SUBLINK SubPlan,
because it will return true if the sub-query returns zero rows, no
matter what the testexpr is.  (Maybe if you could prove the sub-query
does return a row, but I doubt it's worth going there.)

Thanks for pointing this out. You're right. I didn't consider the case
that the subplan produces zero rows.  In this case ALL_SUBLINK will
always return true, and ANY_SUBLINK will always return false.  That
makes ALL_SUBLINK not strict, and ANY_SUBLINK can be strict only at top

* You need to explicitly check the subLinkType; as written this'll
consider EXPR_SUBLINK and so on.  I'm not really on board with
assuming that nothing bad will happen with sublink types other than
the ones the code is expecting.

Yes, I need to check for ANY_SUBLINK and ROWCOMPARE_SUBLINK here.  The
testexpr is only meaningful for ALL/ANY/ROWCOMPARE, and ALL_SUBLINK has

* It's not apparent to me that it's okay to pass down "top_level"
rather than "false".  Maybe it's all right, but it could do with
a comment.