I started looking at a more generic way to fix this. In the attached I'm catching quals being added to baserestrictinfo in distribute_restrictinfo_to_rels() and checking for IS NOT NULL quals on columns defined with NOT NULL.
I did this by adding a new function add_baserestrictinfo_to_rel() which can be the place where we add any future logic to ignore other always-true quals. Perhaps in the future, we can add some logic there to look for quals on partitions which are always true based on the partition constraint.
I think this is a good start. Maybe we can extend it with little effort to cover OR clauses. For an OR clause, we can test its sub-clauses and if one of them is IS NOT NULL qual on a NOT NULL column then we can know that the OR clause is always true.
Maybe we can also test if the qual is always true according to the applicable constraint expressions of the given relation, something that is like the opposite of relation_excluded_by_constraints(). Of course that would require much more efforts.
Another thing I'm wondering is that since we already have the outer-join-aware-Var infrastructure, maybe we can also test whether a IS NOT NULL qual in join clauses is always true. I imagine we need to test whether the Var in the IS NOT NULL qual has an empty varnullingrels besides that the Var is a NOT NULL column.
BTW, with this patch the variable ‘rel’ in function distribute_restrictinfo_to_rels is unused.
Attached is what I have in mind. The patch extends the logic from two points.
* it also checks OR clauses to see if it is always true.
* it also checks for join clauses by additionally testing if the nulling bitmap is empty.
I did not try the logic about testing a qual against the relation's constraints though.
