expr: switch off left push down due to intergration test failed

expression/expr_to_pb_test.go

@@ -1246,11 +1246,6 @@ func TestExprPushDownToTiKV(t *testing.T) {
 			retType:      types.NewFieldType(mysql.TypeString),
 			args:         []Expression{stringColumn, intColumn},
 		},
-		{
-			functionName: ast.Left,
-			retType:      types.NewFieldType(mysql.TypeString),
-			args:         []Expression{stringColumn, intColumn},
-		},
 		{
 			functionName: ast.Sin,
 			retType:      types.NewFieldType(mysql.TypeDouble),

expression/expression.go

@@ -975,7 +975,7 @@ func scalarExprSupportedByTiKV(sf *ScalarFunction) bool {
 		ast.Length, ast.BitLength, ast.Concat, ast.ConcatWS, ast.Replace, ast.ASCII, ast.Hex,
 		ast.Reverse, ast.LTrim, ast.RTrim, ast.Strcmp, ast.Space, ast.Elt, ast.Field,
 		InternalFuncFromBinary, InternalFuncToBinary, ast.Mid, ast.Substring, ast.Substr, ast.CharLength,
-		ast.Right, ast.Left,
+		ast.Right, /* ast.Left */
 
 		// json functions.
 		ast.JSONType, ast.JSONExtract, ast.JSONObject, ast.JSONArray, ast.JSONMerge, ast.JSONSet,

planner/core/integration_test.go

@@ -2771,10 +2771,6 @@ func TestScalarFunctionPushDown(t *testing.T) {
 	tk.MustQuery("explain analyze select /*+read_from_storage(tikv[t])*/ * from t where right(c,1);").
 		CheckAt([]int{0, 3, 6}, rows)
 
-	rows[1][2] = "left(test.t.c, 1)"
-	tk.MustQuery("explain analyze select /*+read_from_storage(tikv[t])*/ * from t where left(c,1);").
-		CheckAt([]int{0, 3, 6}, rows)
-
 	rows[1][2] = "mod(test.t.id, test.t.id)"
 	tk.MustQuery("explain analyze select /*+read_from_storage(tikv[t])*/ * from t where mod(id, id);").
 		CheckAt([]int{0, 3, 6}, rows)