mysql> create table t1 (a varchar(32), b varchar(32), primary key (a,b));
mysql> alter table add index(b);
mysql> insert into t1 values('aaa','bbb');
Query OK, 1 row affected (0.00 sec)

00004000  07 11 7d 29 00 00 00 04  ff ff ff ff ff ff ff ff  |..})............|
00004010  00 00 00 00 ff ee cf d5  45 bf 00 00 00 00 00 00  |........E.......|
00004020  00 00 00 00 09 31 00 02  00 85 80 03 00 00 00 00  |.....1..........|
00004030  00 7f 00 05 00 00 00 01  00 00 00 00 00 01 e0 7e  |...............~|
00004040  00 00 00 00 00 00 00 00  18 1a 00 00 09 31 00 00  |.............1..|
00004050  00 02 02 72 00 00 09 31  00 00 00 02 01 b2 01 00  |...r...1........|
00004060  02 00 1c 69 6e 66 69 6d  75 6d 00 02 00 0b 00 00  |...infimum......|
00004070  73 75 70 72 65 6d 75 6d  03 03 00 00 10 ff f1 62  |supremum.......b|
00004080  62 62 61 61 61 00 00 00  00 00 00 00 00 00 00 00  |bbaaa...........|
00004090  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|