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

Add support for implicitly setting the default value of a column
to null, for cascading tables for both ON DELETE and ON UPDATE.

Previously, if a user set a reference to a column and added a
SET DEFAULT for an ON DELETE/UPDATE cascading action, without an
explicit default expression, CRDB would forbid it. Postgres does not
distinguish between SET NULL and the absence of a default expression.
Thus, the change is to also have CRDB not distinguish between the two
cases.

In the situation where a user creates a table with an explicit
default null via SET NULL, CRDB does not allow the user to create the
table at all. This deviates from how postgres handles it. Postgres
allows the user to create the table but throws an error if a
cascading action occurs that uses the default value. The new changes
now prevent the user from creating a table when they constrain the
columns to be non-nullable, and implicitly set the default to NULL.
This is done to match the behavior for the explicit case of setting
the default to NULL.

Resolves: #38975

Release note (sql change): Columns without an explicit default value now
support foreign keys with the SET DEFAULT action, in the same way as the
SET NULL and SET DEFAULT NULL cases.

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() {