sql: block set default on computed col

This patch blocks setting/dropping a default (even a
null default) on a computed column in the
LSC and DSC.

Fixes: #127522

Release note (bug fix): Setting or dropping a default value
on a computed column is now blocked -- even for null
defaults. Previously, setting or dropping a default value on a
computed column was a no-op.

pkg/sql/alter_table.go

@@ -943,6 +943,25 @@ func applyColumnMutation(
 		return AlterColumnType(ctx, tableDesc, col, t, params, cmds, tn)
 
 	case *tree.AlterTableSetDefault:
+		// If our column is computed, block mixing defaults in entirely.
+		// This check exists here instead of later on during validation because
+		// adding a null default to a computed column should also be blocked, but
+		// is undetectable later on since SET DEFAULT NUL means a nil default
+		// expression.
+		if col.IsComputed() {
+			// Block dropping a computed column "default" as well.
+			if t.Default == nil {
+				return pgerror.Newf(
+					pgcode.Syntax,
+					"column %q of relation %q is a computed column",
+					col.GetName(),
+					tn.ObjectName)
+			}
+			return pgerror.Newf(
+				pgcode.Syntax,
+				"computed column %q cannot also have a DEFAULT expression",
+				col.GetName())
+		}
 		if err := updateNonComputedColExpr(
 			params,
 			tableDesc,

pkg/sql/catalog/tabledesc/validate.go

@@ -1143,7 +1143,7 @@ func (desc *wrapper) validateColumns() error {
 
 		if column.IsComputed() {
 			if column.HasDefault() {
-				return pgerror.Newf(pgcode.InvalidTableDefinition,
+				return pgerror.Newf(pgcode.Syntax,
 					"computed column %q cannot also have a DEFAULT expression",
 					column.GetName(),
 				)

pkg/sql/logictest/testdata/logic_test/computed

@@ -1137,7 +1137,7 @@ SELECT * FROM t69327
 f  f
 
 # Regression test for #72881. Computed columns can't have a DEFAULT expr.
-statement error pgcode 42P16 computed column "v" cannot also have a DEFAULT expression
+statement error pgcode 42601 computed column "v" cannot also have a DEFAULT expression
 ALTER TABLE t69327 ALTER COLUMN v SET DEFAULT 'foo'
 
 # Regression test for #69665.Computed columns should be evaluated after
@@ -1184,3 +1184,26 @@ FROM t88128
 ----
 b1    expected_b1  b2    expected_b2
 true  true         true  true
+
+# Regression test for #127522 where we do not properly block adding a default
+# value to a computed column.
+subtest computed_column_with_default
+
+statement ok
+CREATE TABLE foooooo (
+    id INT PRIMARY KEY,
+    x INT NOT NULL,
+    y INT NOT NULL,
+    gen INT AS (x + y) STORED
+);
+
+statement error pgcode 42601 computed column "gen" cannot also have a DEFAULT expression
+ALTER TABLE foooooo ALTER COLUMN gen SET DEFAULT 1;
+
+statement error pgcode 42601 computed column "gen" cannot also have a DEFAULT expression
+ALTER TABLE foooooo ALTER COLUMN gen SET DEFAULT NULL;
+
+statement error pgcode 42601 column "gen" of relation "foooooo" is a computed column
+ALTER TABLE foooooo ALTER COLUMN gen DROP DEFAULT;
+
+subtest end

pkg/sql/schemachanger/scbuild/internal/scbuildstmt/alter_table_alter_column_set_default.go

@@ -32,6 +32,13 @@ func alterTableSetDefault(
 	colID := getColumnIDFromColumnName(b, tbl.TableID, t.Column, true /* required */)
 	col := mustRetrieveColumnElem(b, tbl.TableID, colID)
 	oldDefaultExpr := retrieveColumnDefaultExpressionElem(b, tbl.TableID, colID)
+	colType := mustRetrieveColumnTypeElem(b, tbl.TableID, colID)
+
+	// Block alters on system columns.
+	panicIfSystemColumn(col, t.Column.String())
+
+	// Block disallowed operations on computed columns.
+	panicIfComputedColumn(tn.ObjectName, colType, t.Column.String(), t.Default)
 
 	// For DROP DEFAULT.
 	if t.Default == nil {
@@ -41,9 +48,6 @@ func alterTableSetDefault(
 		return
 	}
 
-	// Block alters on system columns.
-	panicIfSystemColumn(col, t.Column.String())
-
 	// If our target column already has a default expression, we want to drop it first.
 	if oldDefaultExpr != nil {
 		b.Drop(oldDefaultExpr)
@@ -142,3 +146,22 @@ func sanitizeColumnExpression(
 	s := tree.Serialize(typedExpr)
 	return typedExpr, s, nil
 }
+
+// panicIfComputedColumn blocks disallowed operations on computed columns.
+func panicIfComputedColumn(tn tree.Name, col *scpb.ColumnType, colName string, def tree.Expr) {
+	// Block setting a column default if the column is computed.
+	if col.ComputeExpr != nil {
+		// Block dropping a computed col "default" as well.
+		if def == nil {
+			panic(pgerror.Newf(
+				pgcode.Syntax,
+				"column %q of relation %q is a computed column",
+				colName,
+				tn))
+		}
+		panic(pgerror.Newf(
+			pgcode.Syntax,
+			"computed column %q cannot also have a DEFAULT expression",
+			colName))
+	}
+}