expression: implement castTimeAsDuration function pushdown

expression/expr_to_pb_test.go

@@ -1185,6 +1185,11 @@ func TestExprPushDownToFlash(t *testing.T) {
 	pushed, remained = PushDownExprsWithExtraInfo(sc, exprs, client, kv.TiFlash, true)
 	require.Len(t, pushed, len(exprs))
 	require.Len(t, remained, 0)
+
+	// CastTimeAsDuration
+	function, err = NewFunction(mock.NewContext(), ast.Cast, types.NewFieldType(mysql.TypeDatetime), durationColumn)
+	require.NoError(t, err)
+	exprs = append(exprs, function)
 }
 
 func TestExprOnlyPushDownToFlash(t *testing.T) {

expression/expression.go

@@ -1106,6 +1106,8 @@ func scalarExprSupportedByFlash(function *ScalarFunction) bool {
 			tipb.ScalarFuncSig_CastStringAsTime /*, tipb.ScalarFuncSig_CastDurationAsTime, tipb.ScalarFuncSig_CastJsonAsTime*/ :
 			// ban the function of casting year type as time type pushing down to tiflash because of https://github.com/pingcap/tidb/issues/26215
 			return function.GetArgs()[0].GetType().GetType() != mysql.TypeYear
+		case tipb.ScalarFuncSig_CastTimeAsDuration:
+			return retType.GetType() == mysql.TypeDuration
 		}
 	case ast.DateAdd, ast.AddDate:
 		switch function.Function.PbCode() {

planner/core/integration_test.go

@@ -7092,3 +7092,40 @@ func TestEltPushDownToTiFlash(t *testing.T) {
 	}
 	tk.MustQuery("explain select elt(a, b) from t;").CheckAt([]int{0, 2, 4}, rows)
 }
+
+func TestCastTimeAsDurationToTiFlash(t *testing.T) {
+	store, dom := testkit.CreateMockStoreAndDomain(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a date, b datetime(4))")
+	tk.MustExec("insert into t values('2021-10-26', '2021-10-26')")
+	tk.MustExec("insert into t values('2021-10-26', '2021-10-26 11:11:11')")
+	tk.MustExec("insert into t values('2021-10-26', '2021-10-26 11:11:11.111111')")
+	tk.MustExec("insert into t values('2021-10-26', '2021-10-26 11:11:11.123456')")
+	tk.MustExec("insert into t values('2021-10-26', '2021-10-26 11:11:11.999999')")
+
+	tk.MustExec("set @@tidb_allow_mpp=1; set @@tidb_enforce_mpp=1")
+	tk.MustExec("set @@tidb_isolation_read_engines = 'tiflash'")
+
+	// Create virtual tiflash replica info.
+	is := dom.InfoSchema()
+	db, exists := is.SchemaByName(model.NewCIStr("test"))
+	require.True(t, exists)
+	for _, tblInfo := range db.Tables {
+		if tblInfo.Name.L == "t" {
+			tblInfo.TiFlashReplica = &model.TiFlashReplicaInfo{
+				Count:     1,
+				Available: true,
+			}
+		}
+	}
+
+	rows := [][]interface{}{
+		{"TableReader_9", "root", "data:ExchangeSender_8"},
+		{"└─ExchangeSender_8", "mpp[tiflash]", "ExchangeType: PassThrough"},
+		{"  └─Projection_4", "mpp[tiflash]", "cast(test.t.a, time BINARY)->Column#4, cast(test.t.b, time BINARY)->Column#5"},
+		{"    └─TableFullScan_7", "mpp[tiflash]", "keep order:false, stats:pseudo"},
+	}
+	tk.MustQuery("explain select cast(a as time), cast(b as time) from t;").CheckAt([]int{0, 2, 4}, rows)
+}