User Statistics
开启变量：默认是关闭的，它在内存中创建了几个HASH表
set global userstat=on;
set global thread_statistics=on;


查看索引使用情况：可以判断索引是否用到
select * from t where c1='a1' and c2='a2' and c5='a5' order by c2,c3;


SELECT * FROM INFORMATION_SCHEMA.INDEX_STATISTICSWHERE TABLE_NAME='t' and table_schema='test';
+--------------+------------+------------+-----------+
| TABLE_SCHEMA | TABLE_NAME | INDEX_NAME | ROWS_READ |
+--------------+------------+------------+-----------+
| test | t | idx_t_c1|18 |
+--------------+------------+------------+-----------+


ROWS_READ:从索引访问取的行数，是个累积值

显示客户端连接详细信息
SELECT * FROM INFORMATION_SCHEMA.CLIENT_STATISTICS\G;