mysql 系统优化常用sql（5、Performance-Schema）

今天主要介绍如何通过Performance-Schema得到DBA关心的数据，比如哪个SQL执行次数最多，哪个表访问最频繁，哪个锁最热等信息。通过充分利用Performance-Schema表的数据，让DBA更了解DB的运行状态，也更有助于排查定位问题。

这里主要借助了events_statements_summary_by_digest ，这个表保存着许多关键指标，抓取了与每条标准化语句有关的延迟、错误和查询量信息。

-- 查询执行次数最多的SQL

select DIGEST_TEXT,COUNT_STAR,FIRST_SEEN,LAST_SEEN FROM performance_schema.events_statements_summary_by_digest ORDER BY COUNT_STAR DESC

-- 哪类SQL的平均响应时间最多

SELECT DIGEST_TEXT,AVG_TIMER_WAIT FROM performance_schema.events_statements_summary_by_digest ORDER BY AVG_TIMER_WAIT DESC

-- 哪类SQL排序记录数最多

select digest_text,sum_sort_rows from performance_schema.events_statements_summary_by_digest order by count_star desc

-- 哪类SQL扫描记录数最多

select digest_text,sum_rows_examined from  performance_schema.events_statements_summary_by_digest ORDER BY count_star desc

-- 哪类SQL使用临时表最多

select digest_text,sum_created_tmp_tables,sum_created_tmp_disk_tables from  performance_schema.events_statements_summary_by_digest ORDER BY count_star DESC

-- 哪个表物理IO最多？

SELECT file_name, event_name, SUM_NUMBER_OF_BYTES_READ, SUM_NUMBER_OF_BYTES_WRITE FROM `performance_schema`.file_summary_by_instance ORDER BY SUM_NUMBER_OF_BYTES_READ + SUM_NUMBER_OF_BYTES_WRITE DESC;