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这里讲的缓存是数据库本身的缓存，并不是外部缓存例如Redis/Memcache等等。
数据库的数据分为冷数据和热数据库，通俗的讲冷数据是存储在磁盘上不经常查询的数据；而热数据是频繁查询的数据，这部分数据会被缓存到内存中。
频繁访问且返回相同结果集的情况下使用缓存。
偶尔查询一次且间隔时间较长的情况下不要使用缓存。
尺寸较大的结果集不建议使用缓存，因为数据太大太大，缓存不足以存储，会导致频繁载入与销毁，命中率低。
通常数据库默认情况是开启缓存的，也就是说正常的select查询，如果符合缓存规则就会经过缓存。
当一条SQL查询时如果结果集在内存中称作“命中”
通常DBA只能控制数据库缓存是否开启，分配多少内存给缓存使用，过期销毁时间，以及策略等等.
上面我已经说过，通常数据库默认都开启缓存，所以更多的时候我们的操作是禁用缓存。这就需要开发人员来通过特定的SQL操作来控制数据库缓存。
以 MySQL 为例
mysql> show variables like '%query_cache%'; +------------------------------+---------+ | Variable_name | Value | +------------------------------+---------+ | have_query_cache | YES | | query_cache_limit | 1048576 | | query_cache_min_res_unit | 4096 | | query_cache_size | 1048576 | | query_cache_type | OFF | | query_cache_wlock_invalidate | OFF | +------------------------------+---------+ 6 rows in set (0.04 sec)
编辑 my.cnf 文件，加入配置项 query_cache_type=1 然后重启mysql服务
mysql> show variables like '%query_cache%'; +------------------------------+---------+ | Variable_name | Value | +------------------------------+---------+ | have_query_cache | YES | | query_cache_limit | 1048576 | | query_cache_min_res_unit | 4096 | | query_cache_size | 1048576 | | query_cache_type | ON | | query_cache_wlock_invalidate | OFF | +------------------------------+---------+ 6 rows in set (0.00 sec)
query_cache_type | ON 表示缓存已经开启。
默认情况 select 查询操作只要符合数据库缓存规则那么结果集就会被缓存，如果你的数据库没有开启缓存，请参考下面
set session query_cache_type=on; flush tables; show status like 'qcache_q%'; select sql_cache * from member where id=1; show status like 'qcache_q%'; select sql_cache * from member where id=1; show status like 'qcache_q%';
例 4.2. 演示 SQL_CACHE
mysql> flush tables; Query OK, 0 rows affected (0.00 sec) mysql> show status like 'qcache_q%'; +-------------------------+-------+ | Variable_name | Value | +-------------------------+-------+ | Qcache_queries_in_cache | 0 | +-------------------------+-------+ 1 row in set (0.00 sec) mysql> select sql_cache * from member where id=1; +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | id | age | ctime | ip_address | mobile | mtime | name | picture | sex | status | wechat | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | 1 | 1 | 2017-08-24 17:05:43 | 1 | NULL | NULL | 1 | 1 | 1 | Enable | NULL | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ 1 row in set (0.00 sec) mysql> show status like 'qcache_q%'; +-------------------------+-------+ | Variable_name | Value | +-------------------------+-------+ | Qcache_queries_in_cache | 1 | +-------------------------+-------+ 1 row in set (0.01 sec) mysql> select sql_cache * from member where id=1; +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | id | age | ctime | ip_address | mobile | mtime | name | picture | sex | status | wechat | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | 1 | 1 | 2017-08-24 17:05:43 | 1 | NULL | NULL | 1 | 1 | 1 | Enable | NULL | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ 1 row in set (0.00 sec) mysql> show status like 'qcache_q%'; +-------------------------+-------+ | Variable_name | Value | +-------------------------+-------+ | Qcache_queries_in_cache | 1 | +-------------------------+-------+ 1 row in set (0.01 sec)
我们可以看到 Qcache_queries_in_cache 值由0转为1表示缓存已经生效。
这里我们主要讲怎样禁止缓存，使查询出的结果集不进入缓存。
SELECT SQL_NO_CACHE * FROM table where id=xxxx
下面的用法比较安全，切换到其他数据库也能正常工作
SELECT /*!40001 SQL_NO_CACHE */ * FROM table
set session query_cache_type=on; flush tables; show status like 'qcache_q%'; select sql_no_cache * from member where id=1; show status like 'qcache_q%'; select sql_no_cache * from member where id=1; show status like 'qcache_q%';
例 4.3. 演示 SQL_NO_CACHE
mysql> flush tables; Query OK, 0 rows affected (0.00 sec) mysql> show status like 'qcache_q%'; +-------------------------+-------+ | Variable_name | Value | +-------------------------+-------+ | Qcache_queries_in_cache | 0 | +-------------------------+-------+ 1 row in set (0.00 sec) mysql> select sql_no_cache * from member where id=1; +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | id | age | ctime | ip_address | mobile | mtime | name | picture | sex | status | wechat | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | 1 | 1 | 2017-08-24 17:05:43 | 1 | NULL | NULL | 1 | 1 | 1 | Enable | NULL | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ 1 row in set (0.00 sec) mysql> show status like 'qcache_q%'; +-------------------------+-------+ | Variable_name | Value | +-------------------------+-------+ | Qcache_queries_in_cache | 0 | +-------------------------+-------+ 1 row in set (0.00 sec) mysql> select sql_no_cache * from member where id=1; +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | id | age | ctime | ip_address | mobile | mtime | name | picture | sex | status | wechat | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | 1 | 1 | 2017-08-24 17:05:43 | 1 | NULL | NULL | 1 | 1 | 1 | Enable | NULL | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ 1 row in set (0.00 sec) mysql> show status like 'qcache_q%'; +-------------------------+-------+ | Variable_name | Value | +-------------------------+-------+ | Qcache_queries_in_cache | 0 | +-------------------------+-------+ 1 row in set (0.00 sec)
使用 sql_no_cache 查询 Qcache_queries_in_cache 值始终是 0
我们使用 set session query_cache_type=off 可以关闭本次查询缓存。
set session query_cache_type=off; flush tables; show status like 'qcache_q%'; select sql_cache * from member where id=1; show status like 'qcache_q%'; select sql_cache * from member where id=1; show status like 'qcache_q%';
例 4.4. 演示 query_cache_type=off 关闭查询缓存
mysql> set session query_cache_type=off; Query OK, 0 rows affected (0.00 sec) mysql> mysql> flush tables; Query OK, 0 rows affected (0.00 sec) mysql> show status like 'qcache_q%'; +-------------------------+-------+ | Variable_name | Value | +-------------------------+-------+ | Qcache_queries_in_cache | 0 | +-------------------------+-------+ 1 row in set (0.00 sec) mysql> select sql_cache * from member where id=1; +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | id | age | ctime | ip_address | mobile | mtime | name | picture | sex | status | wechat | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | 1 | 1 | 2017-08-24 17:05:43 | 1 | NULL | NULL | 1 | 1 | 1 | Enable | NULL | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ 1 row in set (0.00 sec) mysql> show status like 'qcache_q%'; +-------------------------+-------+ | Variable_name | Value | +-------------------------+-------+ | Qcache_queries_in_cache | 0 | +-------------------------+-------+ 1 row in set (0.00 sec) mysql> select sql_cache * from member where id=1; +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | id | age | ctime | ip_address | mobile | mtime | name | picture | sex | status | wechat | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ | 1 | 1 | 2017-08-24 17:05:43 | 1 | NULL | NULL | 1 | 1 | 1 | Enable | NULL | +----+-----+---------------------+------------+--------+-------+------+---------+------+--------+--------+ 1 row in set (0.00 sec) mysql> show status like 'qcache_q%'; +-------------------------+-------+ | Variable_name | Value | +-------------------------+-------+ | Qcache_queries_in_cache | 0 | +-------------------------+-------+ 1 row in set (0.00 sec)