[MySQL优化案例]系列 -- 无法使用查询缓存

1. 先看当前的 QCACHE, hits 和 in_cache 均为 0

mysql>SHOW GLOBAL STATUS LIKE 'QCACHE%';

+-------------------------+----------+
| Variable_name           | Value    |
+-------------------------+----------+
| Qcache_hits             | 0        |
| Qcache_not_cached       | 1        |
| Qcache_queries_in_cache | 0        |
+-------------------------+----------+

mysql>SELECT * FROM `Weather` WHERE `AddTime` = 1185897600;

2. 第一次查询完之后，再来看下，not_cached 增加了 1， in_cache 变成 1，因为 SHOW GLOBAL STATUS LIKE 'QCACHE%' 本身不会放在 QCACHE 里的

mysql>SHOW GLOBAL STATUS LIKE 'QCACHE%';
+-------------------------+----------+
| Variable_name           | Value    |
+-------------------------+----------+
| Qcache_hits             | 0        |
| Qcache_not_cached       | 2        |
| Qcache_queries_in_cache | 1        |
+-------------------------+----------+

mysql>SELECT * FROM `Weather` WHERE `AddTime` = 1185897600;

3. 第二次查询完后，再来看看，hits 加 1，in_cache 仍为 1

mysql>SHOW GLOBAL STATUS LIKE 'QCACHE%';
+-------------------------+----------+
| Variable_name           | Value    |
+-------------------------+----------+
| Qcache_hits             | 1        |
| Qcache_not_cached       | 3        |
| Qcache_queries_in_cache | 1        |
+-------------------------+----------+

mysql>SELECT * FROM `Weather` WHERE `AddTime` = DATE_FORMAT(NOW(), '%Y-%m-%d');

4. 执行完上面的查询后看一下，QCACHE 没工作，in_cache 仍为 1，not_cached 增加了 2

mysql>SHOW GLOBAL STATUS LIKE 'QCACHE%';
+-------------------------+----------+
| Variable_name           | Value    |
+-------------------------+----------+
| Qcache_hits             | 1        |
| Qcache_not_cached       | 5        |
| Qcache_queries_in_cache | 1        |
+-------------------------+----------+

mysql>SELECT * FROM `Weather` WHERE `AddTime` = DATE_FORMAT(NOW(), '%Y-%m-%d');

5. 再验证一下，跟刚才的结论一样

mysql>SHOW GLOBAL STATUS LIKE 'QCACHE%';
+-------------------------+----------+
| Variable_name           | Value    |
+-------------------------+----------+
| Qcache_hits             | 1        |
| Qcache_not_cached       | 7        |
| Qcache_queries_in_cache | 1        |
+-------------------------+----------+

总结，如果查询语句条件中带有 CURDATE()、CURRENT_TIME()、FOUND_ROWS() 等函数时，则无法试用查询缓存。具体的请查看MySQL手册的“5.13.1.?How the Query Cache Operates”部分；另外，存储过程/存储函数/触发器也无法用到查询缓存。