Re: Fix bug of CHECK constraint enforceability recursion

The new version looks correct, I don't see any logic problem with it,
however, I do have a performance question:

+ /*
+ * A parent listed in changing_conids is being changed by the
+ * same ALTER, but it may not have been updated yet. For
+ * regular inheritance, recurse upward to check whether an
+ * equivalent enforced parent outside the ALTER will make it
+ * remain enforced. Partitions cannot have multiple parents,
+ * so they do not need this check.
+ */
+ if (!rel->rd_rel->relispartition &&
+ list_member_oid(changing_conids, parentcon->oid))

Shouldn't the parent lookup use some form of caching? Otherwise we'll
end up reevaluating the same parents multiple times. I'm not sure if
it is needed or not, how much of a performance impact this can have in
a real-world server.