I have two tables: comments(id(primary key), author, ip(index)) and visitors(id(primary key), date_time, ip(index))

I want to join them like that:

SELECT visitors.date_time
FROM comments
LEFT JOIN visitors ON ( comments.ip = visitors.ip ) 
WHERE comments.author =  'author'
LIMIT 10

It works, but very slow.. In EXPLAIN it shows that it doesn't use the index on the visitors table:

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  comments    ref author  author  78  const   9660    Using where
1   SIMPLE  visitors    ALL NULL    NULL    NULL    NULL    8033     

Any ideas? Thanks!