MySql Index Problem using two tables

[jeffsbaker]

I was wondering if someone could help me with a mysql indexing problem when using two tables.
The index of 'username' for this code works fine:

EXPLAIN SELECT count( * ) AS num_messages 
FROM messages, users 
WHERE messages.username = 'johndoe' 
AND users.username = 'johndoe' 
AND messages.sent_date >= users.last_activity

Here is the explain:

id select_type table type possible_keys key key_len ref rows Extra 
1 SIMPLE  users ref username  username 53 const 1 Using where 
1 SIMPLE  messages ref username  username 53 const 195 Using where

But in the following select statement the index of 'comments.page' which corresponds to the users.id is not working:

EXPLAIN SELECT count( * ) AS num_comments 
FROM comments, users 
WHERE users.username = 'johndoe' 
AND comments.page = users.id 
AND comments.sent_date >= users.last_activity

Here is the explain:

id select_type table type possible_keys key key_len ref rows Extra 
1 SIMPLE users ref PRIMARY,username username 53 const 1 Using where 
1 SIMPLE comments ALL page NULL NULL NULL 52 Range checked for each record (index map: 0x2)

Does anyone know why MySql is not using the index key 'page' and how I can get it to use the index?
Thanks

[eelixduppy]

Try using a foreign key. For example:

ALTER TABLE  
  users 
ADD FOREIGN KEY 
  (comments_page) 
REFERENCES comments(page) 

[jeffsbaker]

Thanks, but the foreign key does not work because the database is using MYISAM tables.