another related bug, in master.
drop table if exists notnull_tbl1;
CREATE TABLE notnull_tbl1 (c0 int not null, c1 int);
ALTER TABLE notnull_tbl1 ADD CONSTRAINT Q PRIMARY KEY(c0, c1);
\d+ notnull_tbl1
ALTER TABLE notnull_tbl1 ALTER c0 DROP NOT NULL;
ALTER TABLE notnull_tbl1 ALTER c1 DROP NOT NULL;
"ALTER TABLE notnull_tbl1 ALTER c0 DROP NOT NULL;"
should fail?
Yeah, it should fail as before, because c0 is primary key.
In master, although c0's pg_attribute.attnotnull is still true, but its not-null constraint has been deleted
in dropconstraint_internal().
If we drop column c1 after dropping c0 not null, the primary key will be dropped indirectly.
And now you can see c0 is still not-null if you do \d+ notnull_tbl1. But it will report error "not found not-null"
if you alter c0 drop not null.
postgres=# ALTER TABLE notnull_tbl1 ALTER c0 DROP NOT NULL;
ALTER TABLE
postgres=# \d+ notnull_tbl1
Table "public.notnull_tbl1"
Column | Type | Collation | Nullable | Default | Storage | Compression | Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
c0 | integer | | not null | | plain | | |
c1 | integer | | not null | | plain | | |
Indexes:
"q" PRIMARY KEY, btree (c0, c1)
Access method: heap
postgres=# alter table notnull_tbl1 drop c1;
ALTER TABLE
postgres=# \d notnull_tbl1
Table "public.notnull_tbl1"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
c0 | integer | | not null |
postgres=# alter table notnull_tbl1 alter c0 drop not null;
ERROR: could not find not-null constraint on column "c0", relation "notnull_tbl1"