Allow DEFAULT keyword in UPDATE for generated columns (issue #9438)

This commit fixes issue #9438 where updating a generated column with
the DEFAULT keyword was incorrectly rejected. The previous validation
logic prevented any UPDATE of generated columns without checking if
the value being assigned was DEFAULT.

Changes:
- Modified validation in dml.go to allow DEFAULT expressions for generated columns
- For generated columns, DEFAULT now correctly uses the generated expression
- Added comprehensive test case covering various UPDATE scenarios
- Maintains existing validation that rejects non-DEFAULT values for generated columns

MySQL behavior:
- `UPDATE t SET generated_col = DEFAULT` should succeed (now works)
- `UPDATE t SET generated_col = value` should fail (still works)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: elianddb

enginetest/queries/generated_columns.go

@@ -95,6 +95,47 @@ var GeneratedColumnTests = []ScriptTest{
 			},
 		},
 	},
+	{
+		Name: "generated column with DEFAULT in UPDATE clause (issue #9438)",
+		SetUpScript: []string{
+			"create table t (i int primary key, j int generated always as (i + 10))",
+			"insert into t (i) values (1), (2), (3)",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query:    "select * from t order by i",
+				Expected: []sql.Row{{1, 11}, {2, 12}, {3, 13}},
+			},
+			{
+				Query:    "update t set j = default",
+				Expected: []sql.Row{{NewUpdateResult(3, 0)}}, // 3 rows matched, 0 changed (values already correct)
+			},
+			{
+				Query:    "select * from t order by i",
+				Expected: []sql.Row{{1, 11}, {2, 12}, {3, 13}}, // Values should remain the same
+			},
+			{
+				Query:    "update t set i = 5 where i = 1", // This should update both i and j (through generation)
+				Expected: []sql.Row{{NewUpdateResult(1, 1)}},
+			},
+			{
+				Query:    "select * from t order by i",
+				Expected: []sql.Row{{2, 12}, {3, 13}, {5, 15}}, // j should be updated to i + 10 = 15
+			},
+			{
+				Query:    "update t set j = default where i = 5", // Explicit DEFAULT on specific row
+				Expected: []sql.Row{{NewUpdateResult(1, 0)}}, // 1 row matched, 0 changed (value already correct)
+			},
+			{
+				Query:    "select * from t where i = 5",
+				Expected: []sql.Row{{5, 15}}, // Value should still be correct
+			},
+			{
+				Query:       "update t set j = 99", // Should still fail for non-DEFAULT values
+				ExpectedErr: sql.ErrGeneratedColumnValue,
+			},
+		},
+	},
 	{
 		Name: "generated column with DEFAULT in VALUES clause (issue #9428)",
 		SetUpScript: []string{

sql/planbuilder/dml.go

@@ -291,16 +291,28 @@ func (b *Builder) assignmentExprsToExpressions(inScope *scope, e ast.AssignmentE
 			colIdx := tableSch.IndexOfColName(gf.Name())
 			// TODO: during trigger parsing the table in the node is unresolved, so we need this additional bounds check
 			//  This means that trigger execution will be able to update generated columns
-			// Prevent update of generated columns
-			if colIdx >= 0 && tableSch[colIdx].Generated != nil {
+			
+			// Check if this is a DEFAULT expression for a generated column
+			isDefaultExpr := false
+			if _, ok := updateExpr.Expr.(*ast.Default); ok {
+				isDefaultExpr = true
+			}
+			
+			// Prevent update of generated columns, but allow DEFAULT
+			if colIdx >= 0 && tableSch[colIdx].Generated != nil && !isDefaultExpr {
 				err := sql.ErrGeneratedColumnValue.New(tableSch[colIdx].Name, inScope.node.(sql.NameableNode).Name())
 				b.handleErr(err)
 			}
 
 			// Replace default with column default from resolved schema
-			if _, ok := updateExpr.Expr.(*ast.Default); ok {
+			if isDefaultExpr {
 				if colIdx >= 0 {
-					innerExpr = expression.WrapExpression(tableSch[colIdx].Default)
+					// For generated columns, use the generated expression as the default
+					if tableSch[colIdx].Generated != nil {
+						innerExpr = expression.WrapExpression(tableSch[colIdx].Generated)
+					} else {
+						innerExpr = expression.WrapExpression(tableSch[colIdx].Default)
+					}
 				}
 			}
 		}