MDEV-16489 when lead() returns null on a datetime field, the result is treated...

MDEV-16489 when lead() returns null on a datetime field, the result is treated as the literal string '[NULL]'

mysql-test/main/win.result

@@ -3317,3 +3317,20 @@ DROP TABLE t1,t2;
 #
 # Start of 10.3 tests
 #
+#
+# MDEV-16489 when lead() returns null on a datetime field, the result is treated as the literal string '[NULL]'
+#
+CREATE TABLE t1 (d datetime);
+INSERT INTO t1 VALUES ('2018-01-01 00:00:00'),('2018-02-01 00:00:00');
+SELECT *, LEAD(d) OVER (ORDER BY d) AS x FROM t1;
+d	x
+2018-01-01 00:00:00	2018-02-01 00:00:00
+2018-02-01 00:00:00	NULL
+DROP TABLE t1;
+CREATE TABLE t1 (d time);
+INSERT INTO t1 VALUES ('00:00:01'),('00:00:02');
+SELECT *, LEAD(d) OVER (ORDER BY d) AS x FROM t1;
+d	x
+00:00:01	00:00:02
+00:00:02	NULL
+DROP TABLE t1;

mysql-test/main/win.test

@@ -2086,3 +2086,17 @@ DROP TABLE t1,t2;
 --echo #
 --echo # Start of 10.3 tests
 --echo #
+
+--echo #
+--echo # MDEV-16489 when lead() returns null on a datetime field, the result is treated as the literal string '[NULL]'
+--echo #
+
+CREATE TABLE t1 (d datetime);
+INSERT INTO t1 VALUES ('2018-01-01 00:00:00'),('2018-02-01 00:00:00');
+SELECT *, LEAD(d) OVER (ORDER BY d) AS x FROM t1;
+DROP TABLE t1;
+
+CREATE TABLE t1 (d time);
+INSERT INTO t1 VALUES ('00:00:01'),('00:00:02');
+SELECT *, LEAD(d) OVER (ORDER BY d) AS x FROM t1;
+DROP TABLE t1;

sql/item_windowfunc.cc

@@ -444,7 +444,7 @@ bool Item_sum_hybrid_simple::get_date(MYSQL_TIME *ltime, ulonglong fuzzydate)
 {
   DBUG_ASSERT(fixed == 1);
   if (null_value)
-    return 0;
+    return true;
   bool retval= value->get_date(ltime, fuzzydate);
   if ((null_value= value->null_value))
     DBUG_ASSERT(retval == true);