expression: fixed json type for truncate

expression/builtin_math.go

@@ -1743,6 +1743,10 @@ func (c *truncateFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 	if argTp == types.ETTimestamp || argTp == types.ETDatetime || argTp == types.ETDuration || argTp == types.ETString {
 		argTp = types.ETReal
 	}
+	// MySQL compatibility
+	if argTp == types.ETJson {
+		argTp = types.ETInt
+	}
 
 	bf := newBaseBuiltinFuncWithTp(ctx, args, argTp, argTp, types.ETInt)
 
@@ -1762,6 +1766,8 @@ func (c *truncateFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 		sig = &builtinTruncateRealSig{bf}
 	case types.ETDecimal:
 		sig = &builtinTruncateDecimalSig{bf}
+	default:
+		return nil, errIncorrectArgs.GenWithStackByArgs("truncate")
 	}
 
 	return sig, nil

expression/integration_test.go

@@ -519,6 +519,9 @@ func (s *testIntegrationSuite2) TestMathBuiltin(c *C) {
 	result = tk.MustQuery(`select truncate(d, -1), truncate(d, 1), truncate(d, -2), truncate(d, 2) from t;`)
 	result.Check(testkit.Rows("10 12.3 0 12.34"))
 
+	result = tk.MustQuery(`select truncate(json_array(), -1), truncate("cascasc", 1);`)
+	result.Check(testkit.Rows("0 0"))
+
 	// for pow
 	result = tk.MustQuery("SELECT POW('12', 2), POW(1.2e1, '2.0'), POW(12, 2.0);")
 	result.Check(testkit.Rows("144 144 144"))