MySQL Reference Manual - 4.0.20

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5.10 The MySQL Query Cache

From version 4.0.1 on, 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 the identical query is received later, the server retrieves the results from the query cache rather than parsing and executing the 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 generate a lot of dynamic pages based on database content.

Note: It doesn't work in an environment where you have many mysqld servers updating the same MyISAM tables.

Some performance data for the query cache follow. These results were generated by running the MySQL benchmark suite on a Linux Alpha 2 x 500MHz with 2GB RAM and a 64MB query cache.

If all the queries you're performing are simple (such as selecting a row from a table with one row), but still differ so that the queries cannot be cached, the overhead for having the query cache active is 13%. This could be regarded as the worst case scenario. In real life, queries tend to be much more complicated, so the overhead is normally significantly lower.
Searches for a single row in a single-row table are 238% faster with the query cache than without it. This can be regarded as close to the minimum speedup to be expected for a query that is cached.

To disable the query cache at server startup, set the query_cache_size system variable to 0. By disabling the query cache code, there is no noticeable overhead. Query cache capabilities can be excluded from the server entirely by using the --without-query-cache option to configure when compiling MySQL.