Use of indexes when ORDER BY'ing

[Matthias Pigulla]

Hi list,

I can't clearly resolve the following question with manual §10.4, but
I'm sure one of you knows ;-) I'd just like to know wheter MySQL can use
two different keys for retrieving and ordering data.

Example: I've got a table of discussion threads, having the properties
id, forum (the board they belong to) and lastpost. I want to get a list
of all threads for a certain forum, ordered DESC by lastpost.

Now, can I have the keys: primary id, key forum, key lastpost to
successfully (ie with indexes) get the result, or will I have to group
the second and third key together, ie key (forum, lastpost)?

Would it make sense to make this key unique by concatenating the id -
unique (forum, lastpost, id)?

Thanks a lot in advance,
Matthias