slow query

Hi,

 

I have a merged table with 35 million records, the below query takes
around 40 mins to return.

 

mysql> explain select distinct userid from mfs ;

+----+-------------+-------+-------+---------------+------------------+-
--------+------+----------+-------------+

| id | select_type | table | type  | possible_keys | key              |
key_len | ref  | rows     | Extra       |

+----+-------------+-------+-------+---------------+------------------+-
--------+------+----------+-------------+

|  1 | SIMPLE      | mfs   | index | NULL          | mfs_userId_Index |
8 | NULL | 35539364 | Using index |

+----+-------------+-------+-------+---------------+------------------+-
--------+------+----------+-------------+

 

There are approx 20K distinct userid are available.

 

The show processlist stays in 'Sending Data' state for the complete
query period (i.e. 40 mins).

 

502 | root | localhost:4166 | webnmsdb | Query   |  361 | Sending data |
select distinct userid from mfs

 

If I add a limit say limit 100 then it returns in 8 secs.

 

I'm using MYSQL 4.1 on Windows.

 

Is there any way to make this query faster?

 

Ila.