Bug#4594: column index make = failed for gbk, but like works

Fix for HEAP+HASH prefix keys.

mysql-test/r/ctype_utf8.result

@@ -560,3 +560,13 @@ select * from t1 where str='str';
 str
 str
 drop table t1;
+create table t1 (
+str varchar(255) character set utf8 not null,
+key str using hash (str(2))
+) engine=heap;
+INSERT INTO t1 VALUES ('str');
+INSERT INTO t1 VALUES ('str2');
+select * from t1 where str='str';
+str
+str
+drop table t1;

mysql-test/t/ctype_utf8.test

@@ -417,3 +417,15 @@ INSERT INTO t1 VALUES ('str');
 INSERT INTO t1 VALUES ('str2');
 select * from t1 where str='str';
 drop table t1;
+
+# the same for HEAP+HASH
+#
+
+create table t1 (
+  str varchar(255) character set utf8 not null,
+  key str using hash (str(2))
+) engine=heap;
+INSERT INTO t1 VALUES ('str');
+INSERT INTO t1 VALUES ('str2');
+select * from t1 where str='str';
+drop table t1;

sql/key.cc

@@ -211,10 +211,17 @@ bool key_cmp_if_same(TABLE *table,const byte *key,uint idx,uint key_length)
       if (!(key_part->key_type & (FIELDFLAG_NUMBER+FIELDFLAG_BINARY+
 				  FIELDFLAG_PACK)))
       {
-	if (my_strnncoll(key_part->field->charset(),
-			 (const uchar*) key, length,
-		         (const uchar*) table->record[0]+key_part->offset,
-			 length))
+        CHARSET_INFO *cs= key_part->field->charset();
+        uint char_length= key_part->length / cs->mbmaxlen;
+        const byte *pos= table->record[0] + key_part->offset;
+        if (length > char_length)
+        {
+          char_length= my_charpos(cs, pos, pos + length, char_length);
+          set_if_smaller(char_length, length);
+        }
+	if (cs->coll->strnncollsp(cs,
+                                  (const uchar*) key, length,
+                                  (const uchar*) pos, char_length))
 	  return 1;
       }
       else if (memcmp(key,table->record[0]+key_part->offset,length))