George Hubka

here is a clipping from the MySQL manual regarding the query cache to help your website performance.

mysql> SHOW VARIABLES LIKE ‘query_cache_size’;
+——————+——-+
| Variable_name | Value |
+——————+——-+
| query_cache_size | 41984 |
+——————+——-+
For the query cache to actually be able to hold any query results, its size must be set larger:
mysql> SET GLOBAL query_cache_size = 1000000;
Query OK, 0 rows affected (0.04 sec)

mysql> SHOW VARIABLES LIKE ‘query_cache_size’;
+——————+——–+
| Variable_name | Value |
+——————+——–+
| query_cache_size | 999424 |
+——————+——–+
1 row in set (0.00 sec)
The query_cache_size will be aligned to the nearest 1024 byte block. The value reported may therefore be different from the value that you set.
If the query cache size is greater than 0, the query_cache_type variable influences how it works. This variable can be set to the following values:
A value of 0 or OFF prevents caching or retrieval of cached results.
A value of 1 or ON allows caching except of those statements that begin with SELECT SQL_NO_CACHE.
A value of 2 or DEMAND causes caching of only those statements that begin with SELECT SQL_CACHE.
Setting the GLOBAL query_cache_type value determines query cache behavior for all clients that connect after the change is made. Individual clients can control cache behavior for their own connection by setting the SESSION query_cache_type value. For example, a client can disable use of the query cache for its own queries like this:
mysql> SET SESSION query_cache_type = OFF;
To control the maximum size of individual query results that can be cached, set the query_cache_limit system variable. The default value is 1MB.
When a query is to be cached, its result (the data sent to the client) is stored in the query cache during result retrieval. Therefore the data usually is not handled in one big chunk. The query cache allocates blocks for storing this data on demand, so when one block is filled, a new block is allocated. Because memory allocation operation is costly (timewise), the query cache allocates blocks with a minimum size given by the query_cache_min_res_unit system variable. When a query is executed, the last result block is trimmed to the actual data size so that unused memory is freed. Depending on the types of queries your server executes, you might find it helpful to tune the value of query_cache_min_res_unit:
The default value of query_cache_min_res_unit is 4KB. This should be adequate for most cases.
If you have a lot of queries with small results, the default block size may lead to memory fragmentation, as indicated by a large number of free blocks. Fragmentation can force the query cache to prune (delete) queries from the cache due to lack of memory. In this case, you should decrease the value of query_cache_min_res_unit. The number of free blocks and queries removed due to pruning are given by the values of the Qcache_free_blocks and Qcache_lowmem_prunes status variables.
If most of your queries have large results (check the Qcache_total_blocks and Qcache_queries_in_cache status variables), you can increase performance by increasing query_cache_min_res_unit. However, be careful to not make it too large (see the previous item).