expression: push down left/right/abs to tiflash (#25018)

expression/expression.go

@@ -999,16 +999,19 @@ func scalarExprSupportedByFlash(function *ScalarFunction) bool {
 	case
 		ast.LogicOr, ast.LogicAnd, ast.UnaryNot, ast.BitNeg, ast.Xor, ast.And, ast.Or,
 		ast.GE, ast.LE, ast.EQ, ast.NE, ast.LT, ast.GT, ast.In, ast.IsNull, ast.Like,
-		ast.Plus, ast.Minus, ast.Div, ast.Mul, /*ast.Mod,*/
+		ast.Plus, ast.Minus, ast.Div, ast.Mul, ast.Abs, /*ast.Mod,*/
 		ast.If, ast.Ifnull, ast.Case,
 		ast.Concat, ast.ConcatWS,
 		ast.Year, ast.Month, ast.Day,
 		ast.DateDiff, ast.TimestampDiff, ast.DateFormat, ast.FromUnixTime,
 		ast.JSONLength:
 		return true
-	case ast.Substr, ast.Substring:
+	case ast.Substr, ast.Substring, ast.Left, ast.Right, ast.CharLength:
 		switch function.Function.PbCode() {
 		case
+			tipb.ScalarFuncSig_LeftUTF8,
+			tipb.ScalarFuncSig_RightUTF8,
+			tipb.ScalarFuncSig_CharLengthUTF8,
 			tipb.ScalarFuncSig_Substring2ArgsUTF8,
 			tipb.ScalarFuncSig_Substring3ArgsUTF8:
 			return true