[HACKERS] ALTER TABLE parent SET WITHOUT OIDS and the oid column

I suspect the following is a bug:

create table foo (a int) with oids;
CREATE TABLE
create table bar (a int);
CREATE TABLE
alter table bar inherit foo;
ERROR:  table "bar" without OIDs cannot inherit from table "foo" with OIDs

alter table bar set with oids;
ALTER TABLE
alter table bar inherit foo;
ALTER TABLE

alter table foo set without oids;
ERROR:  relation 16551 has non-inherited attribute "oid"

Because:

select attinhcount from pg_attribute where attrelid = 'bar'::regclass and
attname = 'oid';
 attinhcount
-------------
           0
(1 row)

Which also means "oid" can be safely dropped from bar breaking the
invariant that if the parent table has oid column, its child tables must too:

alter table bar drop oid;  -- or, alter table bar set without oids;
ALTER TABLE

Attached patches modifies MergeAttributesIntoExisting() such that we
increment attinhcount not only for user attributes, but also for the oid
system column if one exists.

Thoughts?

Thanks,
Amit

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