From a31fdf7eca7440560934a7ae97c1c5bc82cca291 Mon Sep 17 00:00:00 2001
From: JmPotato <ghzpotato@gmail.com>
Date: Fri, 12 Jun 2020 10:39:47 +0800
Subject: [PATCH] cherry pick #17848 to release-4.0

Signed-off-by: sre-bot <sre-bot@pingcap.com>
---
 expression/builtin_compare_test.go | 12 ++++++++++++
 expression/builtin_compare_vec.go  | 12 +++++++-----
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/expression/builtin_compare_test.go b/expression/builtin_compare_test.go
index 60dd79f15f0d7..dc910d0a4092f 100644
--- a/expression/builtin_compare_test.go
+++ b/expression/builtin_compare_test.go
@@ -290,6 +290,18 @@ func (s *testEvaluatorSuite) TestGreatestLeastFuncs(c *C) {
 			[]interface{}{tm, 123},
 			curTimeInt, int64(123), false, false,
 		},
+		{
+			[]interface{}{tm, "invalid_time_1", "invalid_time_2", tmWithFsp},
+			curTimeWithFspString, "invalid_time_1", false, false,
+		},
+		{
+			[]interface{}{tm, "invalid_time_2", "invalid_time_1", tmWithFsp},
+			curTimeWithFspString, "invalid_time_2", false, false,
+		},
+		{
+			[]interface{}{tm, "invalid_time", nil, tmWithFsp},
+			nil, nil, true, false,
+		},
 		{
 			[]interface{}{duration, "123"},
 			"12:59:59", "123", false, false,
diff --git a/expression/builtin_compare_vec.go b/expression/builtin_compare_vec.go
index 214fe4b2838b3..c1c8cc18c0c48 100644
--- a/expression/builtin_compare_vec.go
+++ b/expression/builtin_compare_vec.go
@@ -742,7 +742,9 @@ func (b *builtinLeastTimeSig) vecEvalString(input *chunk.Chunk, result *chunk.Co
 				if err = handleInvalidTimeError(b.ctx, err); err != nil {
 					return err
 				} else if !findInvalidTime[i] {
-					invalidValue[i] = result.GetString(i)
+					// Make a deep copy here.
+					// Otherwise invalidValue will internally change with result.
+					invalidValue[i] = string(result.GetBytes(i))
 					findInvalidTime[i] = true
 				}
 				continue
@@ -754,12 +756,12 @@ func (b *builtinLeastTimeSig) vecEvalString(input *chunk.Chunk, result *chunk.Co
 	}
 	result.ReserveString(n)
 	for i := 0; i < n; i++ {
-		if findInvalidTime[i] {
-			result.AppendString(invalidValue[i])
-			continue
-		}
 		if dst.IsNull(i) {
 			result.AppendNull()
+			continue
+		}
+		if findInvalidTime[i] {
+			result.AppendString(invalidValue[i])
 		} else {
 			result.AppendString(dstTimes[i].String())
 		}