Fixed bug mdev-4373:

Valgrind complained on usage of uninitialized memory.

mysql-test/r/selectivity.result

@@ -850,4 +850,21 @@ set optimizer_switch=@save_optimizer_switch;
 set optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity;
 DROP TABLE t1,t2;
 set use_stat_tables=@save_use_stat_tables;
+#
+# Bug mdev-4373: condition on a short varchar column   
+#                with optimizer_use_condition_selectivity=3
+#
+set use_stat_tables=PREFERABLY;
+CREATE TABLE t1 (a varchar(1));
+INSERT INTO t1 VALUES ('x'), ('y');
+ANALYZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+FLUSH TABLES;
+set optimizer_use_condition_selectivity=3;
+SELECT * FROM t1 WHERE a <= 'w';
+a
+set optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity;
+DROP TABLE t1;
+set use_stat_tables=@save_use_stat_tables;
 set use_stat_tables=@save_use_stat_tables;

mysql-test/r/selectivity_innodb.result

@@ -858,6 +858,23 @@ set optimizer_switch=@save_optimizer_switch;
 set optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity;
 DROP TABLE t1,t2;
 set use_stat_tables=@save_use_stat_tables;
+#
+# Bug mdev-4373: condition on a short varchar column   
+#                with optimizer_use_condition_selectivity=3
+#
+set use_stat_tables=PREFERABLY;
+CREATE TABLE t1 (a varchar(1));
+INSERT INTO t1 VALUES ('x'), ('y');
+ANALYZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+FLUSH TABLES;
+set optimizer_use_condition_selectivity=3;
+SELECT * FROM t1 WHERE a <= 'w';
+a
+set optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity;
+DROP TABLE t1;
+set use_stat_tables=@save_use_stat_tables;
 set use_stat_tables=@save_use_stat_tables;
 set optimizer_switch=@save_optimizer_switch_for_selectivity_test;
 SET SESSION STORAGE_ENGINE=DEFAULT;

mysql-test/t/selectivity.test

@@ -444,5 +444,28 @@ DROP TABLE t1,t2;
 
 set use_stat_tables=@save_use_stat_tables;
 
+--echo #
+--echo # Bug mdev-4373: condition on a short varchar column   
+--echo #                with optimizer_use_condition_selectivity=3
+--echo #
+
+set use_stat_tables=PREFERABLY;
+
+CREATE TABLE t1 (a varchar(1));
+INSERT INTO t1 VALUES ('x'), ('y');
+
+ANALYZE TABLE t1;
+FLUSH TABLES;
+
+set optimizer_use_condition_selectivity=3;
+
+SELECT * FROM t1 WHERE a <= 'w';
+
+set optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity;
+
+DROP TABLE t1;
+
+set use_stat_tables=@save_use_stat_tables;
+
 
 set use_stat_tables=@save_use_stat_tables;

sql/field.cc

@@ -6202,14 +6202,16 @@ double Field_str::middle_point_pos(Field *min, Field *max)
   uchar minp_prefix[sizeof(ulonglong)];
   uchar maxp_prefix[sizeof(ulonglong)];
   ulonglong mp, minp, maxp;
+  uint dsz= min(field_length, sizeof(ulonglong) * charset()->mbmaxlen);
+  memset(mp_prefix, 0, sizeof(ulonglong));
+  memset(minp_prefix, 0, sizeof(ulonglong));
+  memset(maxp_prefix, 0, sizeof(ulonglong));
   my_strnxfrm(charset(), mp_prefix, sizeof(mp),
-              ptr + length_size(), sizeof(mp) * charset()->mbmaxlen);
+              ptr + length_size(), dsz);
   my_strnxfrm(charset(), minp_prefix, sizeof(minp),
-              min->ptr + length_size(),
-              sizeof(minp) * charset()->mbmaxlen);
+              min->ptr + length_size(), dsz);
   my_strnxfrm(charset(), maxp_prefix, sizeof(maxp),
-              max->ptr + length_size(),
-              sizeof(maxp) * charset()->mbmaxlen);
+              max->ptr + length_size(), dsz);
   mp= char_prefix_to_ulonglong(mp_prefix);
   minp= char_prefix_to_ulonglong(minp_prefix);
   maxp= char_prefix_to_ulonglong(maxp_prefix);