I tried searching through the forums for this but nothing seemed to cover this exactly.

Given a vary simple database:
Code MySQL:
CREATE TABLE IF NOT EXISTS `Friend` (
  `user_id` int unsigned not NULL,
  `friend_id` int unsigned not NULL,
  `comment` varchar(250) default NULL)

Which would make more sense:
-- index (user_id) AND index (friend_id)
or
-- index (user_id, friend_id) AND index (friend_id, user_id)

In theory, each combination can happen only once in the database (with numbers reversed as a separate unique).

So there can be:
1,2,comment
and
2,1,comment

There are queries that look up a user_id and return all friends, and also looking up the friend and returning all the users that have "friended" them.

I have been using just single-column indexes, but was curious if going to multi-column would help anything.

Thanks for opinions and experiences.

/Cyberfunkr