Is it helpful to improve performance by mysql table indexing?
The cache_default table already has an index by CID (as well as indexes by creation and expiry date that allow old entries to be discarded).
Since almost all queries to the table are by CID there's not really any need for other indexes and it's not clear what benefit you would hope to see.
Adding extra indexes to tables does not result in cost-free performance enhancement. Every additional index makes write operations to the table more expensive, so you should not add any new index to a table until you have clear evidence of slow queries resulting from its absence.