In CREATE syntax: KEY a (field_name(10))

10 now means "number of characters", not number of bytes.

mysql-test/r/ctype_mb.result

@@ -19,3 +19,34 @@ SELECT * FROM t1;
 c1	c2	c3
 aaaabbbbcccc	aaaabbbbcccc	aaaabbbbcccc
 DROP TABLE t1;
+CREATE TABLE t1 (a CHAR(4) CHARACTER SET utf8, KEY key_a(a(3)));
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` char(4) character set utf8 default NULL,
+  KEY `key_a` (`a`(3))
+) TYPE=MyISAM CHARSET=latin1
+SHOW KEYS FROM t1;
+Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment
+t1	1	key_a	1	a	A	NULL	9	NULL	YES	BTREE	
+ALTER TABLE t1 CHANGE a a CHAR(4);
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` char(4) default NULL,
+  KEY `key_a` (`a`(3))
+) TYPE=MyISAM CHARSET=latin1
+SHOW KEYS FROM t1;
+Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment
+t1	1	key_a	1	a	A	NULL	3	NULL	YES	BTREE	
+ALTER TABLE t1 CHANGE a a CHAR(4) CHARACTER SET utf8;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` char(4) character set utf8 default NULL,
+  KEY `key_a` (`a`(3))
+) TYPE=MyISAM CHARSET=latin1
+SHOW KEYS FROM t1;
+Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment
+t1	1	key_a	1	a	A	NULL	9	NULL	YES	BTREE	
+DROP TABLE t1;

mysql-test/t/ctype_mb.test

@@ -6,3 +6,14 @@ SHOW CREATE TABLE t1;
 INSERT INTO t1 VALUES ('aaaabbbbccccdddd','aaaabbbbccccdddd','aaaabbbbccccdddd');
 SELECT * FROM t1;
 DROP TABLE t1;
+
+CREATE TABLE t1 (a CHAR(4) CHARACTER SET utf8, KEY key_a(a(3)));
+SHOW CREATE TABLE t1;
+SHOW KEYS FROM t1;
+ALTER TABLE t1 CHANGE a a CHAR(4);
+SHOW CREATE TABLE t1;
+SHOW KEYS FROM t1;
+ALTER TABLE t1 CHANGE a a CHAR(4) CHARACTER SET utf8;
+SHOW CREATE TABLE t1;
+SHOW KEYS FROM t1;
+DROP TABLE t1;

sql/sql_show.cc

@@ -1214,7 +1214,9 @@ store_create_info(THD *thd, TABLE *table, String *packet)
            !(key_info->flags & HA_FULLTEXT)))
       {
         buff[0] = '(';
-        char* end=int10_to_str((long) key_part->length, buff + 1,10);
+        char* end=int10_to_str((long) key_part->length / 
+			       key_part->field->charset()->mbmaxlen,
+			       buff + 1,10);
         *end++ = ')';
         packet->append(buff,(uint) (end-buff));
       }

sql/sql_table.cc

@@ -729,6 +729,9 @@ int mysql_create_table(THD *thd,const char *db, const char *table_name,
       */
       if (key->type == Key::FULLTEXT)
         column->length=test(f_is_blob(sql_field->pack_flag));
+      else
+        column->length*= sql_field->charset->mbmaxlen;
+
       if (f_is_blob(sql_field->pack_flag))
       {
 	if (!(file->table_flags() & HA_BLOB_KEY))
@@ -2099,10 +2102,11 @@ int mysql_alter_table(THD *thd,char *new_db, char *new_name,
       {						// Check if sub key
 	if (cfield->field->type() != FIELD_TYPE_BLOB &&
 	    (cfield->field->pack_length() == key_part_length ||
-	     cfield->length != cfield->pack_length ||
-	     cfield->pack_length <= key_part_length))
+	     cfield->length <= key_part_length / 
+			       key_part->field->charset()->mbmaxlen))
 	  key_part_length=0;			// Use whole field
       }
+      key_part_length /= key_part->field->charset()->mbmaxlen;
       key_parts.push_back(new key_part_spec(cfield->field_name,
 					    key_part_length));
     }