Allow ALTER TABLE .. SET NOT NULL to skip provably unnecessary scans.

If existing CHECK or NOT NULL constraints preclude the presence
of nulls, we need not look to see whether any are present.

Sergei Kornilov, reviewed by Stephen Frost, Ildar Musin, David Rowley,
and by me.

Discussion: http://postgr.es/m/81911511895540@web58j.yandex.ru

src/test/regress/expected/alter_table.out

@@ -1028,6 +1028,8 @@ insert into atacc1 (test2, test) values (1, NULL);
 ERROR:  null value in column "test" violates not-null constraint
 DETAIL:  Failing row contains (null, 1).
 drop table atacc1;
+-- we want check if not null was implied by constraint
+set client_min_messages to 'debug1';
 -- alter table / alter column [set/drop] not null tests
 -- try altering system catalogs, should fail
 alter table pg_class alter column relname drop not null;
@@ -1043,15 +1045,19 @@ ERROR:  relation "non_existent" does not exist
 -- test checking for null values and primary key
 create table atacc1 (test int not null);
 alter table atacc1 add constraint "atacc1_pkey" primary key (test);
+DEBUG:  ALTER TABLE / ADD PRIMARY KEY will create implicit index "atacc1_pkey" for table "atacc1"
+DEBUG:  building index "atacc1_pkey" on table "atacc1" serially
 alter table atacc1 alter column test drop not null;
 ERROR:  column "test" is in a primary key
 alter table atacc1 drop constraint "atacc1_pkey";
 alter table atacc1 alter column test drop not null;
 insert into atacc1 values (null);
 alter table atacc1 alter test set not null;
+DEBUG:  verifying table "atacc1"
 ERROR:  column "test" contains null values
 delete from atacc1;
 alter table atacc1 alter test set not null;
+DEBUG:  verifying table "atacc1"
 -- try altering a non-existent column, should fail
 alter table atacc1 alter bar set not null;
 ERROR:  column "bar" of relation "atacc1" does not exist
@@ -1065,6 +1071,54 @@ alter table myview alter column test set not null;
 ERROR:  "myview" is not a table or foreign table
 drop view myview;
 drop table atacc1;
+-- set not null verified by constraints
+create table atacc1 (test_a int, test_b int);
+insert into atacc1 values (null, 1);
+-- constraint not cover all values, should fail
+alter table atacc1 add constraint atacc1_constr_or check(test_a is not null or test_b < 10);
+DEBUG:  verifying table "atacc1"
+alter table atacc1 alter test_a set not null;
+DEBUG:  verifying table "atacc1"
+ERROR:  column "test_a" contains null values
+alter table atacc1 drop constraint atacc1_constr_or;
+-- not valid constraint, should fail
+alter table atacc1 add constraint atacc1_constr_invalid check(test_a is not null) not valid;
+alter table atacc1 alter test_a set not null;
+DEBUG:  verifying table "atacc1"
+ERROR:  column "test_a" contains null values
+alter table atacc1 drop constraint atacc1_constr_invalid;
+-- with valid constraint
+update atacc1 set test_a = 1;
+alter table atacc1 add constraint atacc1_constr_a_valid check(test_a is not null);
+DEBUG:  verifying table "atacc1"
+alter table atacc1 alter test_a set not null;
+DEBUG:  existing constraints on column "atacc1"."test_a" are sufficient to prove that it does not contain nulls
+delete from atacc1;
+insert into atacc1 values (2, null);
+alter table atacc1 alter test_a drop not null;
+-- test multiple set not null at same time
+-- test_a checked by atacc1_constr_a_valid, test_b should fail by table scan
+alter table atacc1 alter test_a set not null, alter test_b set not null;
+DEBUG:  existing constraints on column "atacc1"."test_a" are sufficient to prove that it does not contain nulls
+DEBUG:  verifying table "atacc1"
+ERROR:  column "test_b" contains null values
+-- commands order has no importance
+alter table atacc1 alter test_b set not null, alter test_a set not null;
+DEBUG:  verifying table "atacc1"
+ERROR:  column "test_b" contains null values
+-- valid one by table scan, one by check constraints
+update atacc1 set test_b = 1;
+alter table atacc1 alter test_b set not null, alter test_a set not null;
+DEBUG:  verifying table "atacc1"
+alter table atacc1 alter test_a drop not null, alter test_b drop not null;
+-- both column has check constraints
+alter table atacc1 add constraint atacc1_constr_b_valid check(test_b is not null);
+DEBUG:  verifying table "atacc1"
+alter table atacc1 alter test_b set not null, alter test_a set not null;
+DEBUG:  existing constraints on column "atacc1"."test_b" are sufficient to prove that it does not contain nulls
+DEBUG:  existing constraints on column "atacc1"."test_a" are sufficient to prove that it does not contain nulls
+drop table atacc1;
+reset client_min_messages;
 -- test inheritance
 create table parent (a int);
 create table child (b varchar(255)) inherits (parent);

src/test/regress/sql/alter_table.sql

@@ -776,6 +776,9 @@ insert into atacc1 (test2, test) values (2, 3);
 insert into atacc1 (test2, test) values (1, NULL);
 drop table atacc1;
 
+-- we want check if not null was implied by constraint
+set client_min_messages to 'debug1';
+
 -- alter table / alter column [set/drop] not null tests
 -- try altering system catalogs, should fail
 alter table pg_class alter column relname drop not null;
@@ -809,6 +812,43 @@ drop view myview;
 
 drop table atacc1;
 
+-- set not null verified by constraints
+create table atacc1 (test_a int, test_b int);
+insert into atacc1 values (null, 1);
+-- constraint not cover all values, should fail
+alter table atacc1 add constraint atacc1_constr_or check(test_a is not null or test_b < 10);
+alter table atacc1 alter test_a set not null;
+alter table atacc1 drop constraint atacc1_constr_or;
+-- not valid constraint, should fail
+alter table atacc1 add constraint atacc1_constr_invalid check(test_a is not null) not valid;
+alter table atacc1 alter test_a set not null;
+alter table atacc1 drop constraint atacc1_constr_invalid;
+-- with valid constraint
+update atacc1 set test_a = 1;
+alter table atacc1 add constraint atacc1_constr_a_valid check(test_a is not null);
+alter table atacc1 alter test_a set not null;
+delete from atacc1;
+
+insert into atacc1 values (2, null);
+alter table atacc1 alter test_a drop not null;
+-- test multiple set not null at same time
+-- test_a checked by atacc1_constr_a_valid, test_b should fail by table scan
+alter table atacc1 alter test_a set not null, alter test_b set not null;
+-- commands order has no importance
+alter table atacc1 alter test_b set not null, alter test_a set not null;
+
+-- valid one by table scan, one by check constraints
+update atacc1 set test_b = 1;
+alter table atacc1 alter test_b set not null, alter test_a set not null;
+
+alter table atacc1 alter test_a drop not null, alter test_b drop not null;
+-- both column has check constraints
+alter table atacc1 add constraint atacc1_constr_b_valid check(test_b is not null);
+alter table atacc1 alter test_b set not null, alter test_a set not null;
+drop table atacc1;
+
+reset client_min_messages;
+
 -- test inheritance
 create table parent (a int);
 create table child (b varchar(255)) inherits (parent);