qcache

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qcache

高性能MySQL这本书中关于查询缓存有一段这样的描述：

Cache invalidations can happen because of fragmentation, insufficient memory, or
data modifications. If you have allocated enough memory to the cache and tuned the
query_cache_min_res_unit value properly, most cache invalidations should be due to
data modifications. You can see how many queries have modified data by examining
the Com_* status variables (Com_update, Com_delete, and so forth), and you can see
how many queries have been invalidated due to low memory by checking the Qcache_
lowmem_prunes status variable.

既然没有命中，肯定要有com_select，读之后要加到缓存中，要有qcache_inserts，既然是这样，我不解的是应该一次com_select就对应一次qcache_inserts阿，为什么qcache_inserts会比com_select小的多呢？原来是这样的：

The total number of SELECT queries is given by this formula:

Com_select + Qcache_hits+ queries with errors found by parser

The Com_select value is given by this formula:

Qcache_inserts + Qcache_not_cached + queries with errors found during the column-privileges check

总的select查询数等于com_select(没命中) + qcache_hits(命中) + 解析错误的，其中的com_select等于qcache_inserts(失效) + qcache_not_cache + 权限检查错误的。也就是说qcache_inserts这个计数不是表示没被缓存而进行的读，而是缓存失效而进行的读，没被缓存和缓存失效是两个概念，分别计数，但都会引起com_select。

参考：

.0/en/query-cache-status-and-maintenance.html

.html

还是要认真读官方文档阿。