Re: Proving IS NOT NULL inference for ScalarArrayOpExpr's

James Coleman <jtc331(at)gmail(dot)com> writes:
> On Fri, Mar 1, 2019 at 5:28 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I also tweaked it to recognize the case where we can prove the
>> array, rather than the scalar, to be null. I'm not sure how useful
>> that is in practice, but it seemed weird that we'd exploit that
>> only if we can also prove the scalar to be null.

> Just for my own understanding: I thought the "if
> (arrayconst->constisnull)" took care of the array constant being null?

Yeah, but only after we've already matched the subexpr to the LHS,
otherwise we'd not be bothering to determine the array's size.

On the other hand, if for some reason we know that the array side
is NULL, we can conclude that the ScalarArrayOp returns NULL
independently of what the LHS is.

> I don't see a check on the scalar node / lhs. I do see you added a
> check for the entire clause being null, but I'm not sure I understand
> when that would occur (unless it's only in the recursive case?)

Yeah, it's to handle the case where we run into a constant NULL
below a strict node. Typically, that doesn't happen because
eval_const_expressions would have const-folded the upper node, but
it's not impossible given all the cases that clause_is_strict_for
can recurse through now. (The coverage bot does show that code being
reached, although perhaps that only occurs for null-below-ScalarArrayOp,
in which case it might be dead if we were to make eval_const_expressions
smarter.)

regards, tom lane