Bug#12891:

  UNION doesn't return DISTINCT result for multi-byte characters
hp_hash.c:
  This piece of code was pretty wrong, looks
  like no necessary changes were made after
  cut-and-paste from fixed length segment processing.
  Itroduced two new variables safe_length1 and
  safe_length2 to remember the original lengths.
  Fixing my_charpos and set_if_smaller calls
  to pass correct parameters.
ctype_utf8.result, ctype_utf8.test:
  adding test case

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

@@ -1023,3 +1023,15 @@ 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

@@ -857,3 +857,14 @@ 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;