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1. show profile分析SQL
Mysql从5.0.37版本开始增加了对 show profiles 和 show profile 语句的支持。show profiles 能够
在做SQL优化时帮助我们了解时间都耗费到哪里去了。
通过 have_profiling 参数，能够看到当前MySQL是否支持profile：
select @@have_profiling; set profiling=1; -- 开启profiling 开关；
通过profile，我们能够更清楚地了解SQL执行的过程。
首先，我们可以执行一系列的操作：
show databases; use mydb13_optimize; show tables; select * from user
where id < 2; select count(*) from user;
执行完上述命令之后，再执行show profiles 指令， 来查看SQL语句执行的耗时：
show profiles;
通过show profile for query query_id 语句可以查看到该SQL执行过程中每个线程的状态和消耗的
时间：
show profile for query 8;
在获取到最消耗时间的线程状态后，MySQL支持进一步选择all、cpu、block io 、context switch、
page faults等明细类型类查看MySQL在使用什么资源上耗费了过高的时间。
例如，选择查看CPU的耗费时间 ：
show profile cpu for query 133;
2. trace分析优化器执行计划
MySQL5.6提供了对SQL的跟踪trace, 通过trace文件能够进一步了解为什么优化器选择A计划, 而不
是选择B计划：
打开trace，设置格式为 JSON，并设置trace最大能够使用的内存大小，避免解析过程中因为默认
内存过小而不能够完整展示：
SET optimizer_trace="enabled=on",end_markers_in_json=on; set
optimizer_trace_max_mem_size=1000000;
执行SQL语句 ：
select * from user where uid < 2;
最后， 检查information_schema.optimizer_trace就可以知道MySQL是如何执行SQL的 ：
select * from information_schema.optimizer_trace\G;
