ddl: remove unnecessary locking when adding an index (#29772)

ddl/column.go

@@ -1427,7 +1427,7 @@ func (w *updateColumnWorker) cleanRowMap() {
 	}
 }
 
-// BackfillDataInTxn will backfill the table record in a transaction, lock corresponding rowKey, if the value of rowKey is changed.
+// BackfillDataInTxn will backfill the table record in a transaction.
 func (w *updateColumnWorker) BackfillDataInTxn(handleRange reorgBackfillTask) (taskCtx backfillTaskContext, errInTxn error) {
 	oprStartTime := time.Now()
 	errInTxn = kv.RunInNewTxn(context.Background(), w.sessCtx.GetStore(), true, func(ctx context.Context, txn kv.Transaction) error {

ddl/index.go

@@ -1293,8 +1293,8 @@ func (w *addIndexWorker) batchCheckUniqueKey(txn kv.Transaction, idxRecords []*i
 	return nil
 }
 
-// BackfillDataInTxn will backfill table index in a transaction, lock corresponding rowKey, if the value of rowKey is changed,
-// indicate that index columns values may changed, index is not allowed to be added, so the txn will rollback and retry.
+// BackfillDataInTxn will backfill table index in a transaction. If the value of rowKey is changed, there must be some other transactions
+// update the row, result in write conflict, so the txn will rollback and retry.
 // BackfillDataInTxn will add w.batchCnt indices once, default value of w.batchCnt is 128.
 func (w *addIndexWorker) BackfillDataInTxn(handleRange reorgBackfillTask) (taskCtx backfillTaskContext, errInTxn error) {
 	failpoint.Inject("errorMockPanic", func(val failpoint.Value) {
@@ -1329,13 +1329,6 @@ func (w *addIndexWorker) BackfillDataInTxn(handleRange reorgBackfillTask) (taskC
 				continue
 			}
 
-			// Lock the row key to notify us that someone delete or update the row,
-			// then we should not backfill the index of it, otherwise the adding index is redundant.
-			err := txn.LockKeys(context.Background(), new(kv.LockCtx), idxRecord.key)
-			if err != nil {
-				return errors.Trace(err)
-			}
-
 			// Create the index.
 			handle, err := w.index.Create(w.sessCtx, txn, idxRecord.vals, idxRecord.handle, idxRecord.rsData)
 			if err != nil {