Since this is working on parse->rtable this will work only for top level tables as against the inherited tables or partitions which may have their own primary key constraints if the parent doesn't have those.
In that case the table have to be specified in from clause otherwise its error
e.g postgres=# CREATE TABLE cities (
name text,
population float,
altitude int
);
CREATE TABLE
postgres=# CREATE TABLE capitals (
id serial primary key,
state char(2)
) INHERITS (cities);
CREATE TABLE
postgres=# EXPLAIN SELECT * FROM cities WHERE id is not null;
ERROR: column "id" does not exist
LINE 1: EXPLAIN SELECT * FROM cities WHERE id is not null;
Even it will not work on the child table because the primary key constraint on the parent table is not in-force in the child table.
This better be done when planning individual relations, plain or join or upper, where all the necessary information is already available with each of the relations and also the quals, derived as well as user specified, are distributed to individual relations where they should be evalutated. My memory is hazy but it might be possible do this while distributing the quals themselves (distribute_qual_to_rels()).
The place where all the necessary information available is on reduce_outer_joins as the comment of the function states but the downside is its will only be inexpensive if the query contains outer join
Said that, to me, this looks more like something we should be able to do at the time of constraint exclusion. But IIRC, we just prove whether constraints refute a qual and not necessarily whether constraints imply a qual, making it redundant, as is required here. E.g. primary key constraint implies key NOT NULL rendering a "key IS NOT NULL" qual redundant. It might be better to test the case when col IS NOT NULL is specified on a column which already has a NOT NULL constraint. That may be another direction to take. We may require much lesser code.
I don’t add NOT NULL constraint optimization to the patch because cached plan is not invalidation in case of a change in NOT NULL constraint
