[PATCH] Allow SJE to recognize GiST-backed temporal primary keys

Hi Hackers,

relation_has_unique_index_for() checks whether join clause equality
operators belong to the index's opfamily via mergeopfamilies. Since
mergeopfamilies only lists btree opfamilies, this check always fails
for GiST-backed unique indexes such as those created by PRIMARY KEY
with WITHOUT OVERLAPS, preventing self-join elimination.

Fix by falling back to op_in_opfamily() when the mergeopfamilies check
fails. The clause is already known to be a mergejoinable equality, so
confirming the operator is registered in the index's opfamily is
sufficient to prove that the index's uniqueness guarantee applies.

Attached a patch to fix this and added corresponding tests.

Thanks,
Satya