Elide not-null constraint checks on child tables during PK creation

We were unnecessarily acquiring AccessExclusiveLock on all child tables
when "ALTER TABLE ONLY sometab ADD PRIMARY KEY" was run on their parent
table, an oversight in commit 14e87ffa5c54.  This caused deadlocks
during pg_restore of partitioned tables.

The reason to acquire the AEL was that we need to verify that child
tables have the involved columns already marked as not-null; but if the
parent table has an inheritable not-null constraint, then all children
must necessarily be in the correct state already, so we can skip the
check, which avoids acquiring the lock.  Reorder the code so that it
works that way.  This doesn't change things in the case where the
constraint doesn't exist, but that case is of lesser importance because
it doesn't occur during parallel pg_restore.

While at it, reword some errmsg() and add errhint() to similar cases in
related but not adjacent code.

Diagnosed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/67469c1c-38bc-7d94-918a-67033f5dd731@gmx.net
Discussion: https://postgr.es/m/2045026.1743801143@sss.pgh.pa.us
Discussion: https://postgr.es/m/1280408.1744650810@sss.pgh.pa.us

src/backend/catalog/pg_constraint.c

@@ -756,7 +756,9 @@ AdjustNotNullInheritance(Oid relid, AttrNumber attnum,
 			ereport(ERROR,
 					errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 					errmsg("cannot change NO INHERIT status of NOT NULL constraint \"%s\" on relation \"%s\"",
-						   NameStr(conform->conname), get_rel_name(relid)));
+						   NameStr(conform->conname), get_rel_name(relid)),
+					errhint("You might need to make the existing constraint inheritable using %s.",
+							"ALTER TABLE ... ALTER CONSTRAINT ... INHERIT"));
 
 		/*
 		 * Throw an error if the existing constraint is NOT VALID and caller
@@ -767,7 +769,8 @@ AdjustNotNullInheritance(Oid relid, AttrNumber attnum,
 					errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 					errmsg("incompatible NOT VALID constraint \"%s\" on relation \"%s\"",
 						   NameStr(conform->conname), get_rel_name(relid)),
-					errhint("You will need to use ALTER TABLE ... VALIDATE CONSTRAINT to validate it."));
+					errhint("You might need to validate it using %s.",
+							"ALTER TABLE ... VALIDATE CONSTRAINT"));
 
 		if (!is_local)
 		{

src/backend/commands/tablecmds.c

@@ -540,6 +540,7 @@ static ObjectAddress ATExecDropColumn(List **wqueue, Relation rel, const char *c
 static void ATPrepAddPrimaryKey(List **wqueue, Relation rel, AlterTableCmd *cmd,
 								bool recurse, LOCKMODE lockmode,
 								AlterTableUtilityContext *context);
+static void verifyNotNullPKCompatible(HeapTuple tuple, const char *colname);
 static ObjectAddress ATExecAddIndex(AlteredTableInfo *tab, Relation rel,
 									IndexStmt *stmt, bool is_rebuild, LOCKMODE lockmode);
 static ObjectAddress ATExecAddStatistics(AlteredTableInfo *tab, Relation rel,
@@ -9438,8 +9439,26 @@ ATExecDropColumn(List **wqueue, Relation rel, const char *colName,
 }
 
 /*
- * Prepare to add a primary key on table, by adding not-null constraints
+ * Prepare to add a primary key on a table, by adding not-null constraints
  * on all columns.
+ *
+ * The not-null constraints for a primary key must cover the whole inheritance
+ * hierarchy (failing to ensure that leads to funny corner cases).  For the
+ * normal case where we're asked to recurse, this routine ensures that the
+ * not-null constraints either exist already, or queues a requirement for them
+ * to be created by phase 2.
+ *
+ * For the case where we're asked not to recurse, we verify that a not-null
+ * constraint exists on each column of each (direct) child table, throwing an
+ * error if not.  Not throwing an error would also work, because a not-null
+ * constraint would be created anyway, but it'd cause a silent scan of the
+ * child table to verify absence of nulls.  We prefer to let the user know so
+ * that they can add the constraint manually without having to hold
+ * AccessExclusiveLock while at it.
+ *
+ * However, it's also important that we do not acquire locks on children if
+ * the not-null constraints already exist on the parent, to avoid risking
+ * deadlocks during parallel pg_restore of PKs on partitioned tables.
  */
 static void
 ATPrepAddPrimaryKey(List **wqueue, Relation rel, AlterTableCmd *cmd,
@@ -9447,42 +9466,13 @@ ATPrepAddPrimaryKey(List **wqueue, Relation rel, AlterTableCmd *cmd,
 					AlterTableUtilityContext *context)
 {
 	Constraint *pkconstr;
+	List	   *children;
+	bool		got_children = false;
 
 	pkconstr = castNode(Constraint, cmd->def);
 	if (pkconstr->contype != CONSTR_PRIMARY)
 		return;
 
-	/*
-	 * If not recursing, we must ensure that all children have a NOT NULL
-	 * constraint on the columns, and error out if not.
-	 */
-	if (!recurse)
-	{
-		List	   *children;
-
-		children = find_inheritance_children(RelationGetRelid(rel),
-											 lockmode);
-		foreach_oid(childrelid, children)
-		{
-			foreach_node(String, attname, pkconstr->keys)
-			{
-				HeapTuple	tup;
-				Form_pg_attribute attrForm;
-
-				tup = SearchSysCacheAttName(childrelid, strVal(attname));
-				if (!tup)
-					elog(ERROR, "cache lookup failed for attribute %s of relation %u",
-						 strVal(attname), childrelid);
-				attrForm = (Form_pg_attribute) GETSTRUCT(tup);
-				if (!attrForm->attnotnull)
-					ereport(ERROR,
-							errmsg("column \"%s\" of table \"%s\" is not marked NOT NULL",
-								   strVal(attname), get_rel_name(childrelid)));
-				ReleaseSysCache(tup);
-			}
-		}
-	}
-
 	/* Verify that columns are not-null, or request that they be made so */
 	foreach_node(String, column, pkconstr->keys)
 	{
@@ -9498,42 +9488,46 @@ ATPrepAddPrimaryKey(List **wqueue, Relation rel, AlterTableCmd *cmd,
 		tuple = findNotNullConstraint(RelationGetRelid(rel), strVal(column));
 		if (tuple != NULL)
 		{
-			Form_pg_constraint conForm = (Form_pg_constraint) GETSTRUCT(tuple);
+			verifyNotNullPKCompatible(tuple, strVal(column));
-
-			/* a NO INHERIT constraint is no good */
-			if (conForm->connoinherit)
-				ereport(ERROR,
-						errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-						errmsg("cannot create primary key on column \"%s\"",
-							   strVal(column)),
-				/*- translator: third %s is a constraint characteristic such as NOT VALID */
-						errdetail("The constraint \"%s\" on column \"%s\", marked %s, is incompatible with a primary key.",
-								  NameStr(conForm->conname), strVal(column), "NO INHERIT"),
-						errhint("You will need to make it inheritable using %s.",
-								"ALTER TABLE ... ALTER CONSTRAINT ... INHERIT"));
-
-			/* an unvalidated constraint is no good */
-			if (!conForm->convalidated)
-				ereport(ERROR,
-						errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-						errmsg("cannot create primary key on column \"%s\"",
-							   strVal(column)),
-				/*- translator: third %s is a constraint characteristic such as NOT VALID */
-						errdetail("The constraint \"%s\" on column \"%s\", marked %s, is incompatible with a primary key.",
-								  NameStr(conForm->conname), strVal(column), "NOT VALID"),
-						errhint("You will need to validate it using %s.",
-								"ALTER TABLE ... VALIDATE CONSTRAINT"));
 
 			/* All good with this one; don't request another */
 			heap_freetuple(tuple);
 			continue;
 		}
+		else if (!recurse)
+		{
+			/*
+			 * No constraint on this column.  Asked not to recurse, we won't
+			 * create one here, but verify that all children have one.
+			 */
+			if (!got_children)
+			{
+				children = find_inheritance_children(RelationGetRelid(rel),
+													 lockmode);
+				/* only search for children on the first time through */
+				got_children = true;
+			}
+
+			foreach_oid(childrelid, children)
+			{
+				HeapTuple	tup;
+
+				tup = findNotNullConstraint(childrelid, strVal(column));
+				if (!tup)
+					ereport(ERROR,
+							errmsg("column \"%s\" of table \"%s\" is not marked NOT NULL",
+								   strVal(column), get_rel_name(childrelid)));
+				/* verify it's good enough */
+				verifyNotNullPKCompatible(tup, strVal(column));
+			}
+		}
 
 		/* This column is not already not-null, so add it to the queue */
 		nnconstr = makeNotNullConstraint(column);
 
 		newcmd = makeNode(AlterTableCmd);
 		newcmd->subtype = AT_AddConstraint;
+		/* note we force recurse=true here; see above */
 		newcmd->recurse = true;
 		newcmd->def = (Node *) nnconstr;
 
@@ -9541,6 +9535,43 @@ ATPrepAddPrimaryKey(List **wqueue, Relation rel, AlterTableCmd *cmd,
 	}
 }
 
+/*
+ * Verify whether the given not-null constraint is compatible with a
+ * primary key.  If not, an error is thrown.
+ */
+static void
+verifyNotNullPKCompatible(HeapTuple tuple, const char *colname)
+{
+	Form_pg_constraint conForm = (Form_pg_constraint) GETSTRUCT(tuple);
+
+	if (conForm->contype != CONSTRAINT_NOTNULL)
+		elog(ERROR, "constraint %u is not a not-null constraint", conForm->oid);
+
+	/* a NO INHERIT constraint is no good */
+	if (conForm->connoinherit)
+		ereport(ERROR,
+				errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				errmsg("cannot create primary key on column \"%s\"", colname),
+		/*- translator: fourth %s is a constraint characteristic such as NOT VALID */
+				errdetail("The constraint \"%s\" on column \"%s\" of table \"%s\", marked %s, is incompatible with a primary key.",
+						  NameStr(conForm->conname), colname,
+						  get_rel_name(conForm->conrelid), "NO INHERIT"),
+				errhint("You might need to make the existing constraint inheritable using %s.",
+						"ALTER TABLE ... ALTER CONSTRAINT ... INHERIT"));
+
+	/* an unvalidated constraint is no good */
+	if (!conForm->convalidated)
+		ereport(ERROR,
+				errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				errmsg("cannot create primary key on column \"%s\"", colname),
+		/*- translator: fourth %s is a constraint characteristic such as NOT VALID */
+				errdetail("The constraint \"%s\" on column \"%s\" of table \"%s\", marked %s, is incompatible with a primary key.",
+						  NameStr(conForm->conname), colname,
+						  get_rel_name(conForm->conrelid), "NOT VALID"),
+				errhint("You might need to validate it using %s.",
+						"ALTER TABLE ... VALIDATE CONSTRAINT"));
+}
+
 /*
  * ALTER TABLE ADD INDEX
  *

src/test/regress/expected/constraints.out

@@ -1042,6 +1042,7 @@ ALTER TABLE ATACC2 INHERIT ATACC1;
 -- can't override
 ALTER TABLE ATACC1 ADD CONSTRAINT ditto NOT NULL a;
 ERROR:  cannot change NO INHERIT status of NOT NULL constraint "a_is_not_null" on relation "atacc2"
+HINT:  You might need to make the existing constraint inheritable using ALTER TABLE ... ALTER CONSTRAINT ... INHERIT.
 -- dropping the NO INHERIT constraint allows this to work
 ALTER TABLE ATACC2 DROP CONSTRAINT a_is_not_null;
 ALTER TABLE ATACC1 ADD CONSTRAINT ditto NOT NULL a;
@@ -1233,8 +1234,8 @@ Indexes:
 create table cnn_pk (a int not null no inherit);
 alter table cnn_pk add primary key (a);
 ERROR:  cannot create primary key on column "a"
-DETAIL:  The constraint "cnn_pk_a_not_null" on column "a", marked NO INHERIT, is incompatible with a primary key.
+DETAIL:  The constraint "cnn_pk_a_not_null" on column "a" of table "cnn_pk", marked NO INHERIT, is incompatible with a primary key.
-HINT:  You will need to make it inheritable using ALTER TABLE ... ALTER CONSTRAINT ... INHERIT.
+HINT:  You might need to make the existing constraint inheritable using ALTER TABLE ... ALTER CONSTRAINT ... INHERIT.
 drop table cnn_pk;
 -- Ensure partitions are scanned for null values when adding a PK
 create table cnn2_parted(a int) partition by list (a);
@@ -1389,14 +1390,15 @@ Not-null constraints:
 -- If we have an invalid constraint, we can't have another
 ALTER TABLE notnull_tbl1 ADD CONSTRAINT nn1 NOT NULL a NOT VALID NO INHERIT;
 ERROR:  cannot change NO INHERIT status of NOT NULL constraint "nn" on relation "notnull_tbl1"
+HINT:  You might need to make the existing constraint inheritable using ALTER TABLE ... ALTER CONSTRAINT ... INHERIT.
 ALTER TABLE notnull_tbl1 ADD CONSTRAINT nn NOT NULL a;
 ERROR:  incompatible NOT VALID constraint "nn" on relation "notnull_tbl1"
-HINT:  You will need to use ALTER TABLE ... VALIDATE CONSTRAINT to validate it.
+HINT:  You might need to validate it using ALTER TABLE ... VALIDATE CONSTRAINT.
 -- cannot add primary key on a column with an invalid not-null
 ALTER TABLE notnull_tbl1 ADD PRIMARY KEY (a);
 ERROR:  cannot create primary key on column "a"
-DETAIL:  The constraint "nn" on column "a", marked NOT VALID, is incompatible with a primary key.
+DETAIL:  The constraint "nn" on column "a" of table "notnull_tbl1", marked NOT VALID, is incompatible with a primary key.
-HINT:  You will need to validate it using ALTER TABLE ... VALIDATE CONSTRAINT.
+HINT:  You might need to validate it using ALTER TABLE ... VALIDATE CONSTRAINT.
 -- ALTER column SET NOT NULL validates an invalid constraint (but this fails
 -- because of rows with null values)
 ALTER TABLE notnull_tbl1 ALTER a SET NOT NULL;
@@ -1502,7 +1504,7 @@ NOTICE:  merging multiple inherited definitions of column "i"
 ALTER TABLE notnull_inhparent ADD CONSTRAINT nn NOT NULL i NOT VALID;
 ALTER TABLE notnull_inhchild ADD CONSTRAINT nn1 NOT NULL i; -- error
 ERROR:  incompatible NOT VALID constraint "nn" on relation "notnull_inhchild"
-HINT:  You will need to use ALTER TABLE ... VALIDATE CONSTRAINT to validate it.
+HINT:  You might need to validate it using ALTER TABLE ... VALIDATE CONSTRAINT.
 EXECUTE get_nnconstraint_info('{notnull_inhparent, notnull_inhchild, notnull_inhgrand}');
       tabname      | conname | convalidated | conislocal | coninhcount 
 -------------------+---------+--------------+------------+-------------
@@ -1567,6 +1569,24 @@ ERROR:  constraint "nn1" conflicts with NOT VALID constraint on child table "pp_
 ALTER TABLE pp_nn_1 VALIDATE CONSTRAINT nn1;
 ALTER TABLE pp_nn ATTACH PARTITION pp_nn_1 FOR VALUES IN (NULL,5); --ok
 DROP TABLE pp_nn;
+-- Try a partition with an invalid constraint and create a PK on the parent.
+CREATE TABLE pp_nn (a int) PARTITION BY HASH (a);
+CREATE TABLE pp_nn_1 PARTITION OF pp_nn FOR VALUES WITH (MODULUS 2, REMAINDER 0);
+ALTER TABLE pp_nn_1 ADD CONSTRAINT nn NOT NULL a NOT VALID;
+ALTER TABLE ONLY pp_nn ADD PRIMARY KEY (a);
+ERROR:  cannot create primary key on column "a"
+DETAIL:  The constraint "nn" on column "a" of table "pp_nn_1", marked NOT VALID, is incompatible with a primary key.
+HINT:  You might need to validate it using ALTER TABLE ... VALIDATE CONSTRAINT.
+DROP TABLE pp_nn;
+-- same as above, but the constraint is NO INHERIT
+CREATE TABLE pp_nn (a int) PARTITION BY HASH (a);
+CREATE TABLE pp_nn_1 PARTITION OF pp_nn FOR VALUES WITH (MODULUS 2, REMAINDER 0);
+ALTER TABLE pp_nn_1 ADD CONSTRAINT nn NOT NULL a NO INHERIT;
+ALTER TABLE ONLY pp_nn ADD PRIMARY KEY (a);
+ERROR:  cannot create primary key on column "a"
+DETAIL:  The constraint "nn" on column "a" of table "pp_nn_1", marked NO INHERIT, is incompatible with a primary key.
+HINT:  You might need to make the existing constraint inheritable using ALTER TABLE ... ALTER CONSTRAINT ... INHERIT.
+DROP TABLE pp_nn;
 -- Create table with NOT NULL INVALID constraint, for pg_upgrade.
 CREATE TABLE notnull_tbl1_upg (a int, b int);
 INSERT INTO notnull_tbl1_upg VALUES (NULL, 1), (NULL, 2), (300, 3);

src/test/regress/expected/inherit.out

@@ -2286,6 +2286,7 @@ DETAIL:  The column has an inherited not-null constraint.
 -- change NO INHERIT status of inherited constraint: no dice, it's inherited
 alter table cc2 add not null a2 no inherit;
 ERROR:  cannot change NO INHERIT status of NOT NULL constraint "nn" on relation "cc2"
+HINT:  You might need to make the existing constraint inheritable using ALTER TABLE ... ALTER CONSTRAINT ... INHERIT.
 -- remove constraint from cc2: no dice, it's inherited
 alter table cc2 alter column a2 drop not null;
 ERROR:  cannot drop inherited constraint "nn" of relation "cc2"
@@ -2509,6 +2510,7 @@ CREATE TABLE inh_nn_parent (a int, NOT NULL a NO INHERIT);
 CREATE TABLE inh_nn_child() INHERITS (inh_nn_parent);
 ALTER TABLE inh_nn_parent ADD CONSTRAINT nna NOT NULL a;
 ERROR:  cannot change NO INHERIT status of NOT NULL constraint "inh_nn_parent_a_not_null" on relation "inh_nn_parent"
+HINT:  You might need to make the existing constraint inheritable using ALTER TABLE ... ALTER CONSTRAINT ... INHERIT.
 ALTER TABLE inh_nn_parent ALTER a SET NOT NULL;
 ERROR:  cannot change NO INHERIT status of NOT NULL constraint "inh_nn_parent_a_not_null" on relation "inh_nn_parent"
 DROP TABLE inh_nn_parent cascade;
@@ -2520,6 +2522,7 @@ CREATE TABLE inh_nn_lvl2 () INHERITS (inh_nn_lvl1);
 CREATE TABLE inh_nn_lvl3 (CONSTRAINT foo NOT NULL a NO INHERIT) INHERITS (inh_nn_lvl2);
 ALTER TABLE inh_nn_lvl1 ADD PRIMARY KEY (a);
 ERROR:  cannot change NO INHERIT status of NOT NULL constraint "foo" on relation "inh_nn_lvl3"
+HINT:  You might need to make the existing constraint inheritable using ALTER TABLE ... ALTER CONSTRAINT ... INHERIT.
 DROP TABLE inh_nn_lvl1, inh_nn_lvl2, inh_nn_lvl3;
 -- Disallow specifying conflicting NO INHERIT flags for the same constraint
 CREATE TABLE inh_nn1 (a int primary key, b int, not null a no inherit);

src/test/regress/sql/constraints.sql

@@ -940,6 +940,20 @@ ALTER TABLE pp_nn_1 VALIDATE CONSTRAINT nn1;
 ALTER TABLE pp_nn ATTACH PARTITION pp_nn_1 FOR VALUES IN (NULL,5); --ok
 DROP TABLE pp_nn;
 
+-- Try a partition with an invalid constraint and create a PK on the parent.
+CREATE TABLE pp_nn (a int) PARTITION BY HASH (a);
+CREATE TABLE pp_nn_1 PARTITION OF pp_nn FOR VALUES WITH (MODULUS 2, REMAINDER 0);
+ALTER TABLE pp_nn_1 ADD CONSTRAINT nn NOT NULL a NOT VALID;
+ALTER TABLE ONLY pp_nn ADD PRIMARY KEY (a);
+DROP TABLE pp_nn;
+
+-- same as above, but the constraint is NO INHERIT
+CREATE TABLE pp_nn (a int) PARTITION BY HASH (a);
+CREATE TABLE pp_nn_1 PARTITION OF pp_nn FOR VALUES WITH (MODULUS 2, REMAINDER 0);
+ALTER TABLE pp_nn_1 ADD CONSTRAINT nn NOT NULL a NO INHERIT;
+ALTER TABLE ONLY pp_nn ADD PRIMARY KEY (a);
+DROP TABLE pp_nn;
+
 -- Create table with NOT NULL INVALID constraint, for pg_upgrade.
 CREATE TABLE notnull_tbl1_upg (a int, b int);
 INSERT INTO notnull_tbl1_upg VALUES (NULL, 1), (NULL, 2), (300, 3);