I wrote:
> This is not a fundamental shortcoming in the system, it's just
> that optimizer/util/predtest.c has not been built out very far
> with respect to BooleanTest nodes.
Oh ... I'd momentarily forgotten that there's already a patch
in the queue to attack that:
https://commitfest.postgresql.org/51/4690/
That's been stalled for lack of review, so if you were interested
in helping out, that'd be great.
regards, tom lane