ddl: fix creating expression in clustered index table (pingcap#26385)

ddl/db_integration_test.go

@@ -2290,6 +2290,11 @@ func (s *testSerialDBSuite1) TestAddExpressionIndex(c *C) {
 
 	tk.MustQuery("select * from t;").Check(testkit.Rows("1 2.1"))
 
+	// Issue #26371
+	tk.MustExec("drop table if exists t1")
+	tk.MustExec("create table t1(a int, b int, primary key(a, b) clustered)")
+	tk.MustExec("alter table t1 add index idx((a+1))")
+
 	// Issue #17111
 	tk.MustExec("drop table if exists t1")
 	tk.MustExec("create table t1 (a varchar(10), b varchar(10));")

ddl/ddl_api.go

@@ -5140,13 +5140,16 @@ func (d *ddl) CreateIndex(ctx sessionctx.Context, ti ast.Ident, keyType ast.Inde
 		return errors.Trace(err)
 	}
 
+	finalColumns := make([]*model.ColumnInfo, len(tblInfo.Columns), len(tblInfo.Columns)+len(hiddenCols))
+	copy(finalColumns, tblInfo.Columns)
+	finalColumns = append(finalColumns, hiddenCols...)
 	// Check before the job is put to the queue.
 	// This check is redundant, but useful. If DDL check fail before the job is put
 	// to job queue, the fail path logic is super fast.
 	// After DDL job is put to the queue, and if the check fail, TiDB will run the DDL cancel logic.
 	// The recover step causes DDL wait a few seconds, makes the unit test painfully slow.
 	// For same reason, decide whether index is global here.
-	indexColumns, err := buildIndexColumns(append(tblInfo.Columns, hiddenCols...), indexPartSpecifications)
+	indexColumns, err := buildIndexColumns(finalColumns, indexPartSpecifications)
 	if err != nil {
 		return errors.Trace(err)
 	}
@@ -5158,7 +5161,7 @@ func (d *ddl) CreateIndex(ctx sessionctx.Context, ti ast.Ident, keyType ast.Inde
 		if err != nil {
 			return err
 		}
-		idxLen, err = indexColumnsLen(tblInfo.Columns, indexColumns)
+		idxLen, err = indexColumnsLen(finalColumns, indexColumns)
 		if err != nil {
 			return err
 		}