Question

Does MySQL cache queries by default or does it only cache when the SQL_CACHE keyword is used in the query? What other things should I keep in mind when using MySQL caching?

Answer 1

The have_query_cache server system variable indicates whether the
query cache is available:

mysql> SHOW VARIABLES LIKE ‘have_query_cache’; | Variable_name |
Value | | have_query_cache | YES | When using a standard MySQL
binary, this value is always YES, even if query caching is disabled.

Several other system variables control query cache operation. These
can be set in an option file or on the command line when starting
mysqld. The query cache system variables all have names that begin
with query_cache_. They are described briefly in Section 5.1.3,
“Server System Variables”, with additional configuration information
given here.

To set the size of the query cache, set the query_cache_size system
variable. Setting it to 0 disables the query cache. The default size
is 0, so the query cache is disabled by default.

Be careful not to set the size of the cache too large. Due to the need
for threads to lock the cache during updates, you may see lock
contention issues with a very large cache.

Quote taken from the MySQL Site http://dev.mysql.com/doc/refman/5.0/en/query-cache-configuration.html