In the previous post, I shared a basic note on MySQL Query Cache.
In this post, I am sharing the details on Query Cache configuration and monitoring.

The first check, Query Cache is available or not.

Please execute below SHOW command:

mysql> SHOW VARIABLES LIKE 'have_query_cache';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| have_query_cache | YES   |
+------------------+-------+

To disable the query cache at server startup, set the query_cache_size system variable to 0. By default, the query cache is disabled.

Using query_cache_size you can set the size of Query Cache which must be larger when you are planning to hold more data into Cache but again this is a task of analysis of your memory usage.

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)

Query Cache with SELECT :

SELECT SQL_CACHE * FROM tbl_users;
SELECT SQL_NO_CACHE * FROM tbl_users;

In the above two select statements, you can find two identifiers.

SQL_CACHE

The query result is cached if it is cacheable and the value of the query_cache_type system variable is ON or DEMAND.

SQL_NO_CACHE

The server does not use the query cache. It neither checks the query cache to see whether the result is already cached nor does it cache the query result.

SHOW VARIABLES LIKE 'query_cache_type';

• A value of 0 or OFF prevents caching or retrieval of cached results.
• A value of 1 or ON enables 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.

To monitor query cache performance, use SHOW STATUS to view the cache status variables:

mysql> SHOW STATUS LIKE 'Qcache%';
+-------------------------+--------+
| Variable_name           | Value  |
+-------------------------+--------+
| Qcache_free_blocks      | 36     |
| Qcache_free_memory      | 138488 |
| Qcache_hits             | 79570  |
| Qcache_inserts          | 27087  |
| Qcache_lowmem_prunes    | 3114   |
| Qcache_not_cached       | 22989  |
| Qcache_queries_in_cache | 415    |
| Qcache_total_blocks     | 912    |
+-------------------------+--------+