MySQL server features a Query Cache . When in use, the query cache stores the text
of a SELECT query together with the corresponding result that was sent to the
client. If an identical query is later received, the server will retrieve the
results from the query cache rather than parsing and executing the same query
again.

NOTE: The query cache does not return stale data. When data is modified, any
relevant entries in the query cache are flushed.

The query cache is extremely useful in an environment where some tables don’t
change very often and you have a lot of identical queries. This is a typical
situation for many web servers that use a lot of dynamic content.

Query Cacheing was added in MySQL 4.0.1