Mysql 分组查询需要 30 多秒才能执行

Question

我正在尝试执行以下查询

select COUNT(*) from tbl_impressions group by spackage 

一开始执行它大约需要 40 秒，但几次后相同的查询会在 2 秒内执行。

有谁知道原因吗？

PS:所有表都已经编入索引。

提前致谢。

Answer 1

发生这种情况的原因如下:

The query cache stores the text of a SELECT statement together with the corresponding result that was sent to the client. If an identical statement is received later, the server retrieves the results from the query cache rather than parsing and executing the statement again. The query cache is shared among sessions, so a result set generated by one client can be sent in response to the same query issued by another client.

更多文档请看这里http://dev.mysql.com/doc/refman/5.1/en/query-cache.html