Just an adding to bstrawson's answer, "mem-cache-d" is a caching mechanism while Redis is a permanent storage but both store data as key-value pair.
Search on a key-value storage(something like Redis or Membase) more like search all the value in a relational database, too slow. If you want do some querying you may need to move to document-oriented NoSQL type DB such as MongoDB or CouchDB which you can do some query part.
Near future you will able to handle couchbase sever 2.0 which will address all your burning issues with NoSQL data querying with newly introduced UnQL and caching(directly derived from the memcached source code)