Disallow merging ONLY constraints in children tables

When creating a child table, or when attaching an existing table as
child of another, we must not allow inheritable constraints to be
merged with non-inheritable ones, because then grandchildren would not
properly get the constraint.  This would violate the grandparent's
expectations.

Bugs noted by Robert Haas.

Author: Nikhil Sontakke

doc/src/sgml/ref/alter_table.sgml

@@ -482,7 +482,11 @@ ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
 
      <para>
       There must also be matching child-table constraints for all
-      <literal>CHECK</literal> constraints of the parent. Currently
+      <literal>CHECK</literal> constraints of the parent, except those
+      marked non-inheritable (that is, created with <literal>ALTER TABLE ONLY</literal>)
+      in the parent, which are ignored; all child-table constraints matched
+      must not be marked non-inheritable.
+      Currently
       <literal>UNIQUE</literal>, <literal>PRIMARY KEY</literal>, and
       <literal>FOREIGN KEY</literal> constraints are not considered, but
       this might change in the future.

src/backend/catalog/heap.c

@@ -2251,6 +2251,8 @@ AddRelationNewConstraints(Relation rel,
  *
  * Returns TRUE if merged (constraint is a duplicate), or FALSE if it's
  * got a so-far-unique name, or throws error if conflict.
+ *
+ * XXX See MergeConstraintsIntoExisting too if you change this code.
  */
 static bool
 MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
@@ -2307,12 +2309,17 @@ MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
 						(errcode(ERRCODE_DUPLICATE_OBJECT),
 				errmsg("constraint \"%s\" for relation \"%s\" already exists",
 					   ccname, RelationGetRelationName(rel))));
-			/* OK to update the tuple */
-			ereport(NOTICE,
-			   (errmsg("merging constraint \"%s\" with inherited definition",
-					   ccname)));
+
 			tup = heap_copytuple(tup);
 			con = (Form_pg_constraint) GETSTRUCT(tup);
+
+			/* If the constraint is "only" then cannot merge */
+			if (con->conisonly)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("constraint \"%s\" conflicts with non-inherited constraint on relation \"%s\"",
+								ccname, RelationGetRelationName(rel))));
+
 			if (is_local)
 				con->conislocal = true;
 			else
@@ -2322,6 +2329,10 @@ MergeWithExistingConstraint(Relation rel, char *ccname, Node *expr,
 				Assert(is_local);
 				con->conisonly = true;
 			}
+			/* OK to update the tuple */
+			ereport(NOTICE,
+					(errmsg("merging constraint \"%s\" with inherited definition",
+							ccname)));
 			simple_heap_update(conDesc, &tup->t_self, tup);
 			CatalogUpdateIndexes(conDesc, tup);
 			break;

src/backend/commands/tablecmds.c

@@ -8818,18 +8818,18 @@ MergeAttributesIntoExisting(Relation child_rel, Relation parent_rel)
  * Check constraints in child table match up with constraints in parent,
  * and increment their coninhcount.
  *
+ * Constraints that are marked ONLY in the parent are ignored.
+ *
  * Called by ATExecAddInherit
  *
  * Currently all constraints in parent must be present in the child. One day we
- * may consider adding new constraints like CREATE TABLE does. We may also want
- * to allow an optional flag on parent table constraints indicating they are
- * intended to ONLY apply to the master table, not to the children. That would
- * make it possible to ensure no records are mistakenly inserted into the
- * master in partitioned tables rather than the appropriate child.
+ * may consider adding new constraints like CREATE TABLE does.
  *
  * XXX This is O(N^2) which may be an issue with tables with hundreds of
  * constraints. As long as tables have more like 10 constraints it shouldn't be
  * a problem though. Even 100 constraints ought not be the end of the world.
+ *
+ * XXX See MergeWithExistingConstraint too if you change this code.
  */
 static void
 MergeConstraintsIntoExisting(Relation child_rel, Relation parent_rel)
@@ -8862,6 +8862,10 @@ MergeConstraintsIntoExisting(Relation child_rel, Relation parent_rel)
 		if (parent_con->contype != CONSTRAINT_CHECK)
 			continue;
 
+		/* if the parent's constraint is marked ONLY, it's not inherited */
+		if (parent_con->conisonly)
+			continue;
+
 		/* Search for a child constraint matching this one */
 		ScanKeyInit(&child_key,
 					Anum_pg_constraint_conrelid,
@@ -8889,6 +8893,14 @@ MergeConstraintsIntoExisting(Relation child_rel, Relation parent_rel)
 								RelationGetRelationName(child_rel),
 								NameStr(parent_con->conname))));
 
+			/* If the constraint is "only" then cannot merge */
+			if (child_con->conisonly)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("constraint \"%s\" conflicts with non-inherited constraint on child table \"%s\"",
+								NameStr(child_con->conname),
+								RelationGetRelationName(child_rel))));
+
 			/*
 			 * OK, bump the child constraint's inheritance count.  (If we fail
 			 * later on, this change will just roll back.)