数据库与缓存

数据库与缓存

摘要: 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。

本文节选自《Netkiller Architect 手札》

第 10 章数据库与缓存

10.1. 什么是数据库缓存？
10.2. 为什么缓存数据呢？
10.3. 什么时候使用数据库缓存
10.4. 涉及缓存的地方有哪些
10.5. 谁来控制数据库缓存
10.6. 怎么控制数据库缓存
- 10.6.1. SQL_CACHE 缓存
- 10.6.2. 禁止缓存 SQL_NO_CACHE
- 10.6.3. 关闭缓存 set session query_cache_type=off

10.1. 什么是数据库缓存？

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。

10.2. 为什么缓存数据呢？

因为频繁查询相同结果集的数据时，每次到磁盘上查找数据是非常耗时的，所以数据库将频繁查询且返回相同结果集的数据放到内存中，可以减少磁盘访问操作。

10.3. 什么时候使用数据库缓存

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”

10.4. 涉及缓存的地方有哪些

数据库本身，查看数据库缓存状态

数据库应用程序接口（ODBC、JDBC......）

10.5. 谁来控制数据库缓存

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。

10.6. 怎么控制数据库缓存

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。

10.6.1. SQL_CACHE 缓存

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 10.1. 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。

10.6.2. 禁止缓存 SQL_NO_CACHE

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 10.2. 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

10.6.3. 关闭缓存 set session query_cache_type=off

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 10.3. 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)