sql: allow cascading action when default is set implicitly to null

pkg/sql/create_table.go

@@ -599,14 +599,17 @@ func ResolveFK(
 		}
 	}
 
-	// Don't add a SET DEFAULT action on an index that has any column that does
-	// not have a DEFAULT expression.
+	// Don't add a SET DEFAULT action on an index that has any column that has
+	// a DEFAULT expression of NULL and a NOT NULL constraint.
 	if d.Actions.Delete == tree.SetDefault || d.Actions.Update == tree.SetDefault {
 		for _, sourceColumn := range srcCols {
-			if sourceColumn.DefaultExpr == nil {
+			// Having a default expression of NULL, and a constraint of NOT NULL is a
+			// contradiction and should never be allowed.
+			if sourceColumn.DefaultExpr == nil && !sourceColumn.Nullable {
 				col := qualifyFKColErrorWithDB(ctx, txn, tbl.TableDesc(), sourceColumn.Name)
 				return pgerror.Newf(pgcode.InvalidForeignKey,
-					"cannot add a SET DEFAULT cascading action on column %q which has no DEFAULT expression", col,
+					"cannot add a SET DEFAULT cascading action on column %q which has a "+
+						"NOT NULL constraint and a NULL default expression", col,
 				)
 			}
 		}

pkg/sql/logictest/testdata/logic_test/fk

@@ -1201,25 +1201,26 @@ CREATE TABLE a (
 );
 
 # Create a table with no DEFAULT expressions column and a SET DEFAULT action.
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.delete_no_default" which has no DEFAULT expression
-CREATE TABLE no_default_table (
+statement ok
+CREATE TABLE delete_no_default_table (
   id INT PRIMARY KEY
  ,delete_no_default INT REFERENCES a ON DELETE SET DEFAULT
 );
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.update_no_default" which has no DEFAULT expression
-CREATE TABLE no_default_table (
+statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.update_no_default_table.update_no_default" which has a NOT NULL constraint and a NULL default expression
+CREATE TABLE update_no_default_table (
   id INT PRIMARY KEY
  ,update_no_default INT NOT NULL REFERENCES a ON UPDATE SET DEFAULT
 );
 
 # Create a table where the primary key has a SET DEFAULT action.
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.primary_key_table.id" which has no DEFAULT expression
-CREATE TABLE primary_key_table (
+# Primary keys are not allowed to be NULL
+statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.primary_key_table_set_default.id" which has a NOT NULL constraint and a NULL default expression
+CREATE TABLE primary_key_table_set_default (
   id INT PRIMARY KEY REFERENCES a ON DELETE SET DEFAULT
 );
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.primary_key_table.id" which has no DEFAULT expression
+statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.primary_key_table.id" which has a NOT NULL constraint and a NULL default expression
 CREATE TABLE primary_key_table (
   id INT PRIMARY KEY REFERENCES a ON UPDATE SET DEFAULT
 );
@@ -1232,12 +1233,12 @@ CREATE TABLE no_default_table (
  ,update_no_default INT
 );
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.delete_no_default" which has no DEFAULT expression
+statement ok
 ALTER TABLE no_default_table ADD CONSTRAINT no_default_delete_set_default
   FOREIGN KEY (delete_no_default) REFERENCES a (id)
   ON DELETE SET DEFAULT;
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.update_no_default" which has no DEFAULT expression
+statement ok
 ALTER TABLE no_default_table ADD CONSTRAINT no_default_update_set_default
   FOREIGN KEY (update_no_default) REFERENCES a (id)
   ON UPDATE SET DEFAULT;
@@ -1253,19 +1254,20 @@ CREATE TABLE primary_key_table (
   id INT PRIMARY KEY
 );
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.primary_key_table.id" which has no DEFAULT expression
+# id is a primary key and thus cannot be NULL
+statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.primary_key_table.id" which has a NOT NULL constraint and a NULL default expression
 ALTER TABLE primary_key_table ADD CONSTRAINT no_default_delete_set_default
   FOREIGN KEY (id) REFERENCES a (id)
   ON DELETE SET DEFAULT;
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.primary_key_table.id" which has no DEFAULT expression
+statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.primary_key_table.id" which has a NOT NULL constraint and a NULL default expression
 ALTER TABLE primary_key_table ADD CONSTRAINT no_default_update_set_default
   FOREIGN KEY (id) REFERENCES a (id)
   ON UPDATE SET DEFAULT;
 
 # Clean up the tables used so far.
 statement ok
-DROP TABLE primary_key_table, a;
+DROP TABLE primary_key_table, delete_no_default_table, a;
 
 # Now test composite foreign keys
 statement ok
@@ -1276,7 +1278,7 @@ CREATE TABLE a (
 );
 
 # Create a table with a column without a DEFAULT expression and a SET DEFAULT action.
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.ref1" which has no DEFAULT expression
+statement ok
 CREATE TABLE no_default_table (
   id INT PRIMARY KEY
  ,ref1 INT
@@ -1285,55 +1287,108 @@ CREATE TABLE no_default_table (
  ,FOREIGN KEY (ref1, ref2) REFERENCES a (id2, id1) ON DELETE SET DEFAULT
 );
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.ref1" which has no DEFAULT expression
-CREATE TABLE no_default_table (
+statement ok
+INSERT INTO a VALUES (1, 2)
+
+statement ok
+INSERT INTO a VALUES (3, 4)
+
+statement ok
+INSERT INTO no_default_table VALUES (6, 2, 1)
+
+query III colnames
+SELECT * FROM no_default_table
+----
+id  ref1  ref2
+6   2     1
+
+statement ok
+DELETE FROM a WHERE id1=1
+
+query III colnames
+SELECT * FROM no_default_table
+----
+id  ref1  ref2
+6   NULL  NULL
+
+statement ok
+CREATE TABLE no_default_table_on_update (
   id INT PRIMARY KEY
  ,ref1 INT
  ,ref2 INT
  ,INDEX (ref1, ref2)
  ,FOREIGN KEY (ref1, ref2) REFERENCES a (id2, id1) ON UPDATE SET DEFAULT
 );
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.ref1" which has no DEFAULT expression
-CREATE TABLE no_default_table (
+statement ok
+INSERT INTO no_default_table_on_update VALUES (0, 4, 3)
+
+query III colnames
+SELECT * FROM no_default_table_on_update
+----
+id  ref1  ref2
+0   4     3
+
+statement ok
+UPDATE a SET id1=33, id2=44 WHERE id1=3;
+
+query III colnames
+SELECT * FROM no_default_table_on_update
+----
+id  ref1  ref2
+0   NULL  NULL
+
+statement ok
+CREATE TABLE no_default_table_ref2_default_on_delete (
   id INT PRIMARY KEY
  ,ref1 INT
  ,ref2 INT DEFAULT 1
  ,INDEX (ref1, ref2)
  ,FOREIGN KEY (ref1, ref2) REFERENCES a (id2, id1) ON DELETE SET DEFAULT
 );
 
-
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.ref1" which has no DEFAULT expression
-CREATE TABLE no_default_table (
+statement ok
+CREATE TABLE no_default_table_ref2_default_on_update (
   id INT PRIMARY KEY
  ,ref1 INT
  ,ref2 INT DEFAULT 1
  ,INDEX (ref1, ref2)
  ,FOREIGN KEY (ref1, ref2) REFERENCES a (id2, id1) ON UPDATE SET DEFAULT
 );
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.ref2" which has no DEFAULT expression
-CREATE TABLE no_default_table (
+statement ok
+CREATE TABLE no_default_table_ref1_default_on_delete (
   id INT PRIMARY KEY
  ,ref1 INT DEFAULT 1
  ,ref2 INT
  ,INDEX (ref1, ref2)
  ,FOREIGN KEY (ref1, ref2) REFERENCES a (id2, id1) ON DELETE SET DEFAULT
 );
 
-statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.no_default_table.ref2" which has no DEFAULT expression
-CREATE TABLE no_default_table (
+statement ok
+CREATE TABLE no_default_table_ref1_default_on_update (
   id INT PRIMARY KEY
  ,ref1 INT DEFAULT 1
  ,ref2 INT
  ,INDEX (ref1, ref2)
  ,FOREIGN KEY (ref1, ref2) REFERENCES a (id2, id1) ON UPDATE SET DEFAULT
 );
 
+# Create a table with a NOT NULL column and a SET NULL action.
+statement error pq: cannot add a SET DEFAULT cascading action on column "test.public.not_null_table.ref1" which has a NOT NULL constraint and a NULL default expression
+CREATE TABLE not_null_table (
+  id INT PRIMARY KEY
+ ,ref1 INT NOT NULL
+ ,ref2 INT NOT NULL
+ ,INDEX (ref1, ref2)
+ ,FOREIGN KEY (ref1, ref2) REFERENCES a (id2, id1) ON DELETE SET DEFAULT
+);
+
 # Clean up after the test.
 statement ok
-DROP TABLE a;
+DROP TABLE a, no_default_table, no_default_table_on_update, no_default_table_ref2_default_on_delete,
+no_default_table_ref2_default_on_update, no_default_table_ref1_default_on_delete,
+no_default_table_ref1_default_on_update
 
 # Test that no column can have more than 1 FK constraint
 subtest column_uniqueness

pkg/sql/logictest/testdata/logic_test/table

@@ -461,7 +461,7 @@ query TT
 SHOW CREATE TABLE test.null_default
 ----
 test.public.null_default  CREATE TABLE null_default (
-                          ts TIMESTAMP NULL DEFAULT NULL,
+                          ts TIMESTAMP NULL,
                           FAMILY "primary" (ts, rowid)
 )
 

pkg/sql/row/cascader.go

@@ -730,6 +730,11 @@ func (c *cascader) updateRows(
 			if err != nil {
 				return nil, nil, nil, 0, err
 			}
+			// If the default expression is nil, treat it as a SET NULL case.
+			if !column.HasDefault() {
+				referencingIndexValuesByColIDs[columnID] = tree.DNull
+				continue
+			}
 			parsedExpr, err := parser.ParseExpr(*column.DefaultExpr)
 			if err != nil {
 				return nil, nil, nil, 0, err

pkg/sql/sqlbase/structured.go

@@ -2867,17 +2867,10 @@ func (desc *ImmutableTableDescriptor) MakeFirstMutationPublic(
 // ColumnNeedsBackfill returns true if adding the given column requires a
 // backfill (dropping a column always requires a backfill).
 func ColumnNeedsBackfill(desc *ColumnDescriptor) bool {
-	// Consider the case where the user explicitly states the default value of a
-	// new column to be NULL. desc.DefaultExpr is not nil, but the string "NULL"
-	if desc.DefaultExpr != nil {
-		if defaultExpr, err := parser.ParseExpr(*desc.DefaultExpr); err != nil {
-			panic(errors.NewAssertionErrorWithWrappedErrf(err,
-				"failed to parse default expression %s", *desc.DefaultExpr))
-		} else if defaultExpr == tree.DNull {
-			return false
-		}
+	if desc.HasNullDefault() {
+		return false
 	}
-	return desc.DefaultExpr != nil || !desc.Nullable || desc.IsComputed()
+	return desc.HasDefault() || !desc.Nullable || desc.IsComputed()
 }
 
 // HasColumnBackfillMutation returns whether the table has any queued column
@@ -3247,6 +3240,19 @@ func (desc *ColumnDescriptor) IsNullable() bool {
 	return desc.Nullable
 }
 
+// HasNullDefault checks that the column descriptor has a default of NULL.
+func (desc *ColumnDescriptor) HasNullDefault() bool {
+	if !desc.HasDefault() {
+		return false
+	}
+	defaultExpr, err := parser.ParseExpr(*desc.DefaultExpr)
+	if err != nil {
+		panic(errors.NewAssertionErrorWithWrappedErrf(err,
+			"failed to parse default expression %s", *desc.DefaultExpr))
+	}
+	return defaultExpr == tree.DNull
+}
+
 // ColID is part of the cat.Column interface.
 func (desc *ColumnDescriptor) ColID() cat.StableID {
 	return cat.StableID(desc.ID)

pkg/sql/sqlbase/structured_test.go

@@ -1310,7 +1310,7 @@ func TestKeysPerRow(t *testing.T) {
 }
 
 func TestColumnNeedsBackfill(t *testing.T) {
-	// Define variable strings here such that we can pass their address below
+	// Define variable strings here such that we can pass their address below.
 	null := "NULL"
 	four := "4:::INT8"
 	// Create Column Descriptors that reflect the definition of a column with a
@@ -1757,3 +1757,38 @@ func TestUpgradeDowngradeFKRepr(t *testing.T) {
 		}
 	}
 }
+
+func TestDefaultExprNil(t *testing.T) {
+	s, conn, _ := serverutils.StartServer(t, base.TestServerArgs{})
+	defer s.Stopper().Stop(context.TODO())
+	if _, err := conn.Exec(`CREATE DATABASE t`); err != nil {
+		t.Fatalf("%+v", err)
+	}
+	t.Run(fmt.Sprintf("%s - %d", "(a INT PRIMARY KEY)", 1), func(t *testing.T) {
+		sqlDB := sqlutils.MakeSQLRunner(conn)
+		// Execute SQL commands with both implicit and explicit setting of the
+		// default expression.
+		sqlDB.Exec(t, `CREATE TABLE t (a INT PRIMARY KEY)`)
+		sqlDB.Exec(t, `INSERT INTO t (a) VALUES (1), (2)`)
+		sqlDB.Exec(t, `ALTER TABLE t ADD COLUMN b INT NULL`)
+		sqlDB.Exec(t, `INSERT INTO t (a) VALUES (3)`)
+		sqlDB.Exec(t, `ALTER TABLE t ADD COLUMN c INT DEFAULT NULL`)
+
+		var descBytes []byte
+		// Grab the most recently created descriptor.
+		row := sqlDB.QueryRow(t,
+			`SELECT descriptor FROM system.descriptor ORDER BY id DESC LIMIT 1`)
+		row.Scan(&descBytes)
+		var desc Descriptor
+		if err := protoutil.Unmarshal(descBytes, &desc); err != nil {
+			t.Fatalf("%+v", err)
+		}
+		// Test and verify that the default expressions of the column descriptors
+		// are all nil.
+		for _, col := range desc.GetTable().Columns {
+			if col.DefaultExpr != nil {
+				t.Errorf("expected Column Default Expression to be 'nil', got %s instead.", *col.DefaultExpr)
+			}
+		}
+	})
+}

pkg/sql/sqlbase/table.go

@@ -168,12 +168,15 @@ func MakeColumnDefDescs(
 		); err != nil {
 			return nil, nil, nil, err
 		}
-		// We keep the type checked expression so that the type annotation
-		// gets properly stored.
-		d.DefaultExpr.Expr = typedExpr
 
-		s := tree.Serialize(d.DefaultExpr.Expr)
-		col.DefaultExpr = &s
+		// Keep the type checked expression so that the type annotation gets
+		// properly stored, only if the default expression is not NULL.
+		// Otherwise we want to keep the default expression nil.
+		if typedExpr != tree.DNull {
+			d.DefaultExpr.Expr = typedExpr
+			s := tree.Serialize(d.DefaultExpr.Expr)
+			col.DefaultExpr = &s
+		}
 	}
 
 	if d.IsComputed() {