How to structure the database - MYSQL performance question

Indexes on:
SELECTed fields
ORDER BY fields
WHERE fields
fields JOINed on

Thanks for the clarification CroNiX

I'm surprised that an index makes a difference on SELECTed fields, do you know why that is?

Is there any harm in 'over-indexing', or I suppose you could call it defensive indexing?  I appreciate there would be an overhead in maintaining the index, but for data that is not highly volatile I can't see that being too much of an issue.  Are there any other performance factors to consider?