ddl: assign table record prefix to start/end key if it is empty (#39683)

close #39641

ddl/backfilling.go

@@ -508,6 +508,7 @@ func (dc *ddlCtx) handleRangeTasks(scheduler *backfillScheduler, t table.Table,
 	// Build reorg tasks.
 	job := reorgInfo.Job
 	for i, keyRange := range kvRanges {
+		startKey := keyRange.StartKey
 		endKey := keyRange.EndKey
 		endK, err := getRangeEndKey(scheduler.jobCtx, dc.store, job.Priority, prefix, keyRange.StartKey, endKey)
 		if err != nil {
@@ -517,11 +518,17 @@ func (dc *ddlCtx) handleRangeTasks(scheduler *backfillScheduler, t table.Table,
 				zap.String("end key", hex.EncodeToString(endKey)), zap.String("current end key", hex.EncodeToString(endK)))
 			endKey = endK
 		}
+		if len(startKey) == 0 {
+			startKey = prefix
+		}
+		if len(endKey) == 0 {
+			endKey = prefix.PrefixNext()
+		}
 
 		task := &reorgBackfillTask{
 			id:              i,
 			physicalTableID: physicalTableID,
-			startKey:        keyRange.StartKey,
+			startKey:        startKey,
 			endKey:          endKey,
 			// If the boundaries overlap, we should ignore the preceding endKey.
 			endInclude: endK.Cmp(keyRange.EndKey) != 0 || i == len(kvRanges)-1}

tests/realtikvtest/addindextest/integration_test.go

@@ -256,3 +256,19 @@ func TestAddIndexIngestGeneratedColumns(t *testing.T) {
 	tk.MustQuery("select * from t;").Check(testkit.Rows("1 1 1 2 1", "2 2 2 4 2", "3 3 3 6 3"))
 	assertLastNDDLUseIngest(4)
 }
+
+func TestAddIndexIngestEmptyTable(t *testing.T) {
+	store := realtikvtest.CreateMockStoreAndSetup(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("drop database if exists addindexlit;")
+	tk.MustExec("create database addindexlit;")
+	tk.MustExec("use addindexlit;")
+	tk.MustExec("create table t (a int);")
+	tk.MustExec(`set global tidb_ddl_enable_fast_reorg=on;`)
+	tk.MustExec("alter table t add index idx(a);")
+
+	rows := tk.MustQuery("admin show ddl jobs 1;").Rows()
+	require.Len(t, rows, 1)
+	jobTp := rows[0][3].(string)
+	require.True(t, strings.Contains(jobTp, "ingest"), jobTp)
+}