expression: wrong result of timestampadd(month,1,date '2024-01-31') (p…

…ingcap#53101) (pingcap#53420)

close pingcap#41052

expression/builtin_time.go

@@ -6187,6 +6187,13 @@ func addUnitToTime(unit string, t time.Time, v float64) (time.Time, bool, error)
 			return tb, true, nil
 		}
 		tb = t.AddDate(0, int(v), 0)
+
+		// For corner case: timestampadd(month,1,date '2024-01-31') = "2024-02-29", timestampadd(month,1,date '2024-01-30') = "2024-02-29"
+		// `tb.Month()` refers to the actual result, `t.Month()+v` refers to the expect result.
+		// Actual result may be greater than expect result, we need to judge and modify it.
+		for int(tb.Month())%12 != (int(t.Month())+int(v))%12 {
+			tb = tb.AddDate(0, 0, -1)
+		}
 	case "QUARTER":
 		if !validAddMonth(v*3, t.Year(), int(t.Month())) {
 			return tb, true, nil

expression/builtin_time_test.go

@@ -2525,6 +2525,18 @@ func TestTimestampAdd(t *testing.T) {
 		{"MINUTE", 1.5, "1995-05-01 00:00:00", "1995-05-01 00:02:00"},
 		{"MINUTE", 1.5, "1995-05-01 00:00:00.000000", "1995-05-01 00:02:00"},
 		{"MICROSECOND", -100, "1995-05-01 00:00:00.0001", "1995-05-01 00:00:00"},
+
+		// issue41052
+		{"MONTH", 1, "2024-01-31", "2024-02-29 00:00:00"},
+		{"MONTH", 1, "2024-01-30", "2024-02-29 00:00:00"},
+		{"MONTH", 1, "2024-01-29", "2024-02-29 00:00:00"},
+		{"MONTH", 1, "2024-01-28", "2024-02-28 00:00:00"},
+		{"MONTH", 1, "2024-10-31", "2024-11-30 00:00:00"},
+		{"MONTH", 3, "2024-01-31", "2024-04-30 00:00:00"},
+		{"MONTH", 15, "2024-01-31", "2025-04-30 00:00:00"},
+		{"MONTH", 10, "2024-10-31", "2025-08-31 00:00:00"},
+		{"MONTH", 1, "2024-11-30", "2024-12-30 00:00:00"},
+		{"MONTH", 13, "2024-11-30", "2025-12-30 00:00:00"},
 	}
 
 	fc := funcs[ast.TimestampAdd]