使用mysql慢查日志监控有效率问题的SQL语句
通过慢查日志发现有问题的SQL
1、查询次数多且每次查询占用时间长的SQL
通常为pt-query-digest分析的前几个查询
2、IO大的SQL
注意pt-query-digest分析中的Rows examine项，既是扫描的行数越多，IO消耗越大
3、未命中索引的SQL
注意pt-query-digest分析中Rows examine和Rows send的对比，如果扫描的行数远大于发送的行数，那命中率太低了。
1、show variables like ‘slow_query_log’
2、设置变量
set global slow_query_log_file='/var/lib/mysql/mysql_slow_20170829.log'; //慢查日志的记录路径，变量路径一定是mysql用户和组有权限，否则设置失败
set global log_queries_not_using_indexes=on; //重点，不用索引的需要重点优化
set global long_query_time=1; //设置0.01秒，设置为0表示任何sql命令都会被记录
3、limit是mysql的语法
select * from table limit m,n
其中m是指记录开始的index，从0开始，表示第一条记录，一共输出n条
select * from table limit n
从索引0开始，一共输出n条
mysql> show variables like 'long_query_time';
+-----------------+----------+
| Variable_name | Value |
+-----------------+----------+
| long_query_time | 0.000000 |
+-----------------+----------+
1 row in set (0.00 sec)
mysql> show variables like 'slow_query_log';
+----------------+-------+
| Variable_name | Value |
+----------------+-------+
| slow_query_log | ON |
+----------------+-------+
1 row in set (0.00 sec)
mysql> show variables like 'slow%';
+---------------------+----------------------------------------+
| Variable_name | Value |
+---------------------+----------------------------------------+
| slow_launch_time | 2 |
| slow_query_log | ON |
| slow_query_log_file | /var/lib/mysql/mysql_slow_20170829.log |
+---------------------+----------------------------------------+
3 rows in set (0.00 sec)
mysql> use sakila;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> show tables;
+----------------------------+
| Tables_in_sakila |
+----------------------------+
| actor |
| actor_info |
| address |
| category |
| city |
| country |
| customer |
| customer_list |
| film |
| film_actor |
| film_category |
| film_list |
| film_text |
| inventory |
| language |
| nicer_but_slower_film_list |
| payment |
| rental |
| sales_by_film_category |
| sales_by_store |
| staff |
| staff_list |
| store |
+----------------------------+
23 rows in set (0.00 sec)
mysql> select * from store limit 1,3;
+----------+------------------+------------+---------------------+
| store_id | manager_staff_id | address_id | last_update |
+----------+------------------+------------+---------------------+
| 2 | 2 | 2 | 2006-02-15 04:57:12 |
+----------+------------------+------------+---------------------+
1 row in set (0.00 sec)
mysql> quit;
Bye
zjy@ubuntu:~$ su parallels
Password:
parallels@ubuntu:/home/zjy$ sudo -i
[sudo] password for parallels:
root@ubuntu:~# tail /var/lib/mysql/mysql_slow_20170829.log
# Time: 2017-08-29T08:37:20.167516Z
# User@Host: root[root] @ localhost [] Id: 17
# Query_time: 0.000435 Lock_time: 0.000190 Rows_sent: 1 Rows_examined: 2
SET timestamp=1503995840;
select * from store limit 1,3;
# Time: 2017-08-29T08:38:45.837452Z
# User@Host: root[root] @ localhost [] Id: 17
# Query_time: 0.000007 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 2
SET timestamp=1503995925;
# administrator command: Quit;
root@ubuntu:~#