expression: implement least/greatest for string function pushdown

expression/expr_to_pb_test.go

@@ -1124,6 +1124,10 @@ func TestExprPushDownToFlash(t *testing.T) {
 	require.NoError(t, err)
 	exprs = append(exprs, function)
 
+	function, err = NewFunction(mock.NewContext(), ast.Greatest, types.NewFieldType(mysql.TypeString), stringColumn, stringColumn)
+	require.NoError(t, err)
+	exprs = append(exprs, function)
+
 	// least
 	function, err = NewFunction(mock.NewContext(), ast.Least, types.NewFieldType(mysql.TypeLonglong), int32Column, intColumn)
 	require.NoError(t, err)
@@ -1132,6 +1136,11 @@ func TestExprPushDownToFlash(t *testing.T) {
 	function, err = NewFunction(mock.NewContext(), ast.Least, types.NewFieldType(mysql.TypeDouble), float32Column, intColumn)
 	require.NoError(t, err)
 	exprs = append(exprs, function)
+
+	function, err = NewFunction(mock.NewContext(), ast.Least, types.NewFieldType(mysql.TypeString), stringColumn, stringColumn)
+	require.NoError(t, err)
+	exprs = append(exprs, function)
+
 	// is true
 	function, err = NewFunction(mock.NewContext(), ast.IsTruthWithoutNull, types.NewFieldType(mysql.TypeLonglong), int32Column)
 	require.NoError(t, err)

expression/expression.go

@@ -1262,7 +1262,7 @@ func scalarExprSupportedByFlash(function *ScalarFunction) bool {
 	case ast.Least, ast.Greatest:
 		switch function.Function.PbCode() {
 		case tipb.ScalarFuncSig_GreatestInt, tipb.ScalarFuncSig_GreatestReal,
-			tipb.ScalarFuncSig_LeastInt, tipb.ScalarFuncSig_LeastReal:
+			tipb.ScalarFuncSig_LeastInt, tipb.ScalarFuncSig_LeastReal, tipb.ScalarFuncSig_LeastString, tipb.ScalarFuncSig_GreatestString:
 			return true
 		}
 	case ast.IsTruthWithNull, ast.IsTruthWithoutNull, ast.IsFalsity:

planner/core/integration_test.go

@@ -7754,6 +7754,39 @@ func TestUnhexPushDownToTiFlash(t *testing.T) {
 	tk.MustQuery("explain select unhex(b) from t;").CheckAt([]int{0, 2, 4}, rows)
 }
 
+func TestLeastGretestStringPushDownToTiFlash(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 varchar(20), b varchar(20))")
+	tk.MustExec("insert into t values('123', '234')")
+	tk.MustExec("set @@tidb_allow_mpp=1; set @@tidb_enforce_mpp=1")
+	tk.MustExec("set @@tidb_isolation_read_engines = 'tiflash'")
+
+	tbl, err := dom.InfoSchema().TableByName(model.CIStr{O: "test", L: "test"}, model.CIStr{O: "t", L: "t"})
+	require.NoError(t, err)
+	// Set the hacked TiFlash replica for explain tests.
+	tbl.Meta().TiFlashReplica = &model.TiFlashReplicaInfo{Count: 1, Available: true}
+
+	rows := [][]interface{}{
+		{"TableReader_9", "root", "MppVersion: 1, data:ExchangeSender_8"},
+		{"└─ExchangeSender_8", "mpp[tiflash]", "ExchangeType: PassThrough"},
+		{"  └─Projection_4", "mpp[tiflash]", "least(test.t.a, test.t.b)->Column#4"},
+		{"    └─TableFullScan_7", "mpp[tiflash]", "keep order:false, stats:pseudo"},
+	}
+	tk.MustQuery("explain select least(a, b) from t;").CheckAt([]int{0, 2, 4}, rows)
+
+	rows = [][]interface{}{
+		{"TableReader_9", "root", "MppVersion: 1, data:ExchangeSender_8"},
+		{"└─ExchangeSender_8", "mpp[tiflash]", "ExchangeType: PassThrough"},
+		{"  └─Projection_4", "mpp[tiflash]", "greatest(test.t.a, test.t.b)->Column#4"},
+		{"    └─TableFullScan_7", "mpp[tiflash]", "keep order:false, stats:pseudo"},
+	}
+	tk.MustQuery("explain select greatest(a, b) from t;").CheckAt([]int{0, 2, 4}, rows)
+}
+
 func TestPartitionTableFallBackStatic(t *testing.T) {
 	store, _ := testkit.CreateMockStoreAndDomain(t)
 	tk := testkit.NewTestKit(t, store)