> known as "covering indexes". The advantage to a covering index is that if
> your data is numeric and in the index, the engine can read the data
All correct, except that the data does not have to be numeric. It must,
however, be completely indexed, not indexed by prefix. That requirement
usually excludes most BLOB/TEXT fields, so you wouldn't normally include
a BLOB/TEXT in a covered index discussion.
Covered indexes work fine with CHAR/VARCHAR and are quite common.
Technical Yahoo - MySQL (Database) Geek