1. Database performance enhancement apparatus, for use in a network having a set of at least one client system each running an application which makes SQL database queries via a network to a particular IP address and port combination, and a plurality of SQL database servers each having access to at least a portion of a database and running software which accepts SQL database queries received at a respective server IP address and port combination, the apparatus comprising a cache memory, and the apparatus being arranged to: receive, through a network port, a first SQL database query made by the set of client systems and directed to the particular IP address and port combination; determine whether the first SQL database query is an SQL write query or a read query, and if the first SQL database query is a read query then also determine whether the first SQL database query is one for which valid response data is already stored in the cache memory, and: in response to determining that the first SQL database query is a write query and not a read query, forward the first SQL database query through a network port to one or more of the servers in the plurality of database servers, and in response to determining that the first SQL database query is a read query and not a write query and that the first SQL database query is one for which valid response data is already stored in the cache memory, return the response data from the cache memory, and in response to determining that the first SQL database query is a read query and not a write query and that the first SQL database query is one for which valid response data is not stored in the cache memory, forward the first SQL database query through a network port to one or more of the servers in the plurality of database servers.