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先说明2点
开启缓存也会带来开销，主要表现在一下方面
读取在查询开始之前必须要检查缓存
如果查询是缓存的，但是不在结果集中，那么产生结果后保存数据会带来一定的开销
向缓存写如数据也会带来开销
有的情况查询缓存不会被缓存，即使你使用 SQL_CACHE也不能缓存
主要一下几个
引用了用户自定义函数
引用了用户自定义变量
以用了存续过程
查询中包含一些实时的系统函数，比如now
引用了临时表
虽然上面说到缓存会带来一些开销
但是缓存对mysql 还是很重要
带来的好处比坏处多
下面讲一下mysql缓存如何使用内存
查询缓存的内存池被分为大小可变的块，每一个快都知道自己的大小，指向前一个和后一个逻辑块和物理块 的指针，服务器启动时初始化查询缓存使用的物理内存，内存池开始咋hi要一个快，她的大小为被配置为缓存的大小减去自身需要的40KB大小。在每次缓存查询结果时，会为查询结果分配一个缓存块，但是服务器无法估计需要缓存的结果的大小，至少会大于等于query_cache_min_res_unit,，服务器在分配时，不会再缓存中一次生成最终结果然后缓存发送到客户端，而是产生一条缓存一条，这样缓存就能保证空间不会在分配时浪费或者不足。分配内存块的速度较慢，因为服务器需要检查可用内存列表并且找打大小合适的快，因此服务器会尽量减少分配次数，当需要缓存结果时，他会构建一个大小至少为最小值的块，并且把结果放在块中，如果块已经满了，但是数据没有完整保存，那么就会产生一个新块并且保存数据，在数据存完后，如果数据块还有空隙，服务器就会剪辑数据块，并且把空间并入剩余的空间中，在其中可能产生碎片。通过紧缩，可以讲碎片空间并入有效的数据空间中。
对于事物引擎，比如innodb，事物会影响缓存，因为脏数据也会让其他引用了这块数据的缓存失效。直到该事物提交或回滚之后，这块数据才会被缓存。
query_cache_min_res_unit：the minimum size (in bytes) for blocks allocated by the query cache. The default value is 4096 (4KB)
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