Merge rkalimullin@bk-internal.mysql.com:/home/bk/mysql-5.0

into  mysql.com:/usr/home/ram/work/5.0.b12917

heap/hp_hash.c

@@ -551,11 +551,13 @@ int hp_rec_key_cmp(HP_KEYDEF *keydef, const byte *rec1, const byte *rec2,
       }
       if (cs->mbmaxlen > 1)
       {
+        uint safe_length1= char_length1;
+        uint safe_length2= char_length2;
         uint char_length= seg->length / cs->mbmaxlen;
-        char_length1= my_charpos(cs, pos1, pos1 + char_length1, char_length1);
-        set_if_smaller(char_length1, seg->length);
-        char_length2= my_charpos(cs, pos2, pos2 + char_length2, char_length2);
-        set_if_smaller(char_length2, seg->length);
+        char_length1= my_charpos(cs, pos1, pos1 + char_length1, char_length);
+        set_if_smaller(char_length1, safe_length1);
+        char_length2= my_charpos(cs, pos2, pos2 + char_length2, char_length);
+        set_if_smaller(char_length2, safe_length2);
       }
 
       if (cs->coll->strnncollsp(seg->charset,

mysql-test/r/ctype_utf8.result

@@ -1027,3 +1027,15 @@ aa
 xxx
 yyy
 DROP TABLE t1;
+set names utf8;
+create table t1 (a char(1)) default character set utf8;
+create table t2 (a char(1)) default character set utf8;
+insert into t1 values('a'),('a'),(0xE38182),(0xE38182);
+insert into t1 values('i'),('i'),(0xE38184),(0xE38184);
+select * from t1 union distinct select * from t2;
+a
+a
+あ
+i
+い
+drop table t1,t2;

mysql-test/t/ctype_utf8.test

@@ -863,3 +863,14 @@ SELECT DISTINCT id FROM t1;
 SELECT DISTINCT id FROM t1 ORDER BY id;
 
 DROP TABLE t1;
+
+#
+# Bug#12891: UNION doesn't return DISTINCT result for multi-byte characters
+#
+set names utf8;
+create table t1 (a char(1)) default character set utf8;
+create table t2 (a char(1)) default character set utf8;
+insert into t1 values('a'),('a'),(0xE38182),(0xE38182);
+insert into t1 values('i'),('i'),(0xE38184),(0xE38184);
+select * from t1 union distinct select * from t2;
+drop table t1,t2;