difference of performance when retrieving different column from resultset

Hi,
i am experiencing a very strange issue when i try to load different column from a result set.

I have a table t :

desc table t;
+-----------------------+-------------------------------------+------+-----+---------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------------+-------------------------------------+------+-----+---------------------+-------+
| sub_id | varchar(20) | NO | PRI | NULL | |
| net_id | int(10) unsigned | YES | | NULL | |
| ipv4 | varchar(20) | YES | MUL | NULL | |
| ipv6 | varchar(40) | YES | | NULL | |
| doc_version | tinyint(4) | YES | | NULL | |
| tier | varchar(30) | YES | | NULL | |
| last_updated_time | timestamp | NO | | 0000-00-00 00:00:00 | |
| last_completed_period | int(10) unsigned | YES | | 0 | |
| state | enum('active','inactive','deleted') | YES | | active | |
+-----------------------+-------------------------------------+------+-----+---------------------+-------+

when i try to retrieve data (sub_id + something else except tier column) from a large resultset i got a very good time:
Load ResultSet took 8 seconds for 892098 rows

But when i try to retrieve data from the tier column ( + sub_id) it could take up to 30 minutes.

Load ResultSet took 1925 seconds for 892098 rows

Both for sub_id and tier I used the method ResultSet.getString(columnName)

Has anyone faced this issue before? Please help me out. Thanks