* 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
level.
* 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
been proven not strict.
* 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.
The 'top_level' param is one point that I'm not very confident about. I've added comments in the v2 patch.
