Caching DB queries

Hello people..

I'm planning on building a caching mod that will cache DB queries in Redis or Memcached for a client that has problems with heavy queries.

My first thought where to modify the DB class to check the cache system on every query that contained a select statement. On second though i'm starting to think if there's any queries that would be bad to cache. Like transactions and orders. Can anyone elaborate if there would be any problems to this?

Yes
caching queries will work only for categories, products and not for orders, cart