一、SQL执行频率
MySQL客户端连接成功后,通过show [session | global] status 命令可以提供服务器状态信息,通过如下指令,可以查看当前数据库的insert,update,dalete,select的访问冰刺
show [global | session] status like "Com_______"; # 七个_ 表示起个通配符
mysql> show global status like 'Com_______';+---------------+-------+| Variable_name | Value |+---------------+-------+| Com_binlog | 0 || Com_commit | 0 || Com_delete | 0 || Com_import | 0 || Com_insert | 0 || Com_repair | 0 || Com_revoke | 0 || Com_select | 4 || Com_signal | 0 || Com_update | 0 || Com_xa_end | 0 |+---------------+-------+11 rows in set (0.00 sec)
说明1:上面的数据库被执行查询4次
二、慢查询日志
慢查询日志记录了所有执行时间超过指定参数(long_query_time 单位:秒,默认10秒)的所有SQL语句的日志,Mysql的慢查询日志默认没有开启,需要在Mysql的配置文件中(通常在/etc/my.cnf)中配置如下信息:
可以使用一下语句查询慢查询是否开启
mysql> show variables like 'slow_query_log';+----------------+-------+| Variable_name | Value |+----------------+-------+| slow_query_log | OFF |+----------------+-------+1 row in set (0.01 sec)
说明:慢查询默认是关闭的
# 开启慢查询slow_query_log=1# 设置慢查询的时间long_query_time=2
再次查询
mysql> show variables like 'slow_query_log';+----------------+-------+| Variable_name | Value |+----------------+-------+| slow_query_log | ON |+----------------+-------+1 row in set (0.00 sec)
慢日志文件通常指mysql的安装目录里面的data文件夹中。
三、profile 3.1 show profiles
可以查看每一条SQL的耗时基本情况
mysql> show profiles;+----------+-------------+-----------------------------------------------------------------------+| Query_ID | Duration | Query |+----------+-------------+-----------------------------------------------------------------------+| 11 | 0.00020000 | SELECT DATABASE() || 12 | 0.00029000 | SELECT DATABASE() || 13 | 0.00040900 | SELECT DATABASE() || 14 | 0.00145600 | show databases || 15 | 0.00279800 | show tables || 16 | 12.28066100 | select * from account_transaction || 17 | 0.00166700 | select * from account_transaction where id = 1 || 18 | 6.01525200 | select * from account_transaction where trade_no="164126925202017539" || 19 | 6.64749300 | select * from account_transaction where trade_no="164126925202017539" || 20 | 5.39658800 | select * from account_transaction where trade_no="164126923751014167" || 21 | 0.00067300 | select * from account_transaction where id=100 || 22 | 0.00046900 | select * from account_transaction where id=1000 || 23 | 0.00045200 | select * from account_transaction where id=10000 || 24 | 0.00052900 | select * from account_transaction where id=100000 || 25 | 0.00038300 | select * from account_transaction where id=20000 |+----------+-------------+-----------------------------------------------------------------------+15 rows in set, 1 warning (0.00 sec)
说明1:第16条查询全部数据花费了12.28秒,第17条根据id查询只花费了0.001秒,第18条通过普通字段查询花费了6.00秒
说明2:SQL中能不做全量查询就不要做全量查询。
说明3:SQL中能通过id查询就不要通过其他字段查询,因为毕竟其他字段的查询还是会根据二级索引查到id,再根据id查询到具体的数据的。
3.2 have_profiling
参数have_profiling能够看到当前mysql是否支持profile操作:
mysql> select @@have_profiling;+------------------+| @@have_profiling |+------------------+| YES |+------------------+1 row in set, 1 warning (0.00 sec)
说明1:这里的YES只是说明该版本的mysql是支持profile操作的,但是不代表profile操作是开始的,仅代表有这个功能而已!
默认profiling是关闭的,可以通过set语句在session/global级别开启profiling;
mysql> select @@profiling;+-------------+| @@profiling |+-------------+| 0 |+-------------+1 row in set, 1 warning (0.01 sec)
3.3 开启profiling
mysql> set profiling=1;Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> select @@profiling;+-------------+| @@profiling |+-------------+| 1 |+-------------+1 row in set, 1 warning (0.00 sec)
3.4 查看指定SQL耗时
通过带query_id的SQL语句各个阶段的耗时情况
show profile for query query_id;
mysql> show profile for query 20;+--------------------------------+----------+| Status | Duration |+--------------------------------+----------+| starting | 0.000083 || Executing hook on transaction | 0.000007 || starting | 0.000007 || checking permissions | 0.000006 || Opening tables | 0.000107 || init | 0.000012 || System lock | 0.000010 || optimizing | 0.000012 || statistics | 0.000025 || preparing | 0.000041 || executing | 5.393642 || end | 0.000016 || query end | 0.000005 || waiting for handler commit | 0.000009 || closing tables | 0.000009 || freeing items | 0.002130 || logging slow query | 0.000426 || cleaning up | 0.000041 |+--------------------------------+----------+18 rows in set, 1 warning (0.01 sec)
3.5 查看指定SQL的CPU使用情况
show profile cpu for query query_id
mysql> show profile cpu for query 20;+--------------------------------+----------+----------+------------+| Status | Duration | CPU_user | CPU_system |+--------------------------------+----------+----------+------------+| starting | 0.000083 | 0.000072 | 0.000009 || Executing hook on transaction | 0.000007 | 0.000003 | 0.000004 || starting | 0.000007 | 0.000006 | 0.000002 || checking permissions | 0.000006 | 0.000004 | 0.000002 || Opening tables | 0.000107 | 0.000058 | 0.000017 || init | 0.000012 | 0.000005 | 0.000006 || System lock | 0.000010 | 0.000008 | 0.000002 || optimizing | 0.000012 | 0.000010 | 0.000002 || statistics | 0.000025 | 0.000023 | 0.000001 || preparing | 0.000041 | 0.000027 | 0.000014 || executing | 5.393642 | 2.294837 | 0.151005 || end | 0.000016 | 0.000007 | 0.000009 || query end | 0.000005 | 0.000003 | 0.000001 || waiting for handler commit | 0.000009 | 0.000009 | 0.000001 || closing tables | 0.000009 | 0.000008 | 0.000002 || freeing items | 0.002130 | 0.000037 | 0.000063 || logging slow query | 0.000426 | 0.000034 | 0.000175 || cleaning up | 0.000041 | 0.000021 | 0.000018 |+--------------------------------+----------+----------+------------+18 rows in set, 1 warning (0.00 sec)
四、explain执行计划
explain 或者 desc 命令获取Mysql如何执行select 语句的信息,包括在select 语句在执行过程中表如何连接,及连接的顺序
4.1 语法
explain/desc select 字段列表 from 表名 where 条件;
4.2 示例
mysql> select * from account_transaction where id=100;+-----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+| id | trade_no | type | method | time | payment | out_trade_no | amount | balance | trader_staff_id | operator_staff_id | device_id | remark |+-----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+| 100 | 156384784634000449 | TOP_UP | CASH | 2019-07-23 02:10:46.929559 | LOCAL_ACCOUNT | | 10000 | 10000 | 449 | 11 | 7 | |+-----+--------------------+--------+--------+----------------------------+---------------+--------------+--------+---------+-----------------+-------------------+-----------+--------+1 row in set (0.00 sec)mysql> explain select * from account_transaction where id=100;+----+-------------+---------------------+------------+-------+---------------+---------+---------+-------+------+----------+-------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+---------------------+------------+-------+---------------+---------+---------+-------+------+----------+-------+| 1 | SIMPLE | account_transaction | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |+----+-------------+---------------------+------------+-------+---------------+---------+---------+-------+------+----------+-------+1 row in set, 1 warning (0.00 sec)
4.3 explain字段含义
参数id:select查询的序列号,表示查询语句中的执行顺序,如果id相同,执行顺序从上到下,id不同,值越大,越先执行
mysql> select s.*, c.* from student s, course c,student_course sc where s.id=sc.student_id and c.id = sc.course_id;+----+--------+----+--------+| id | name | id | name |+----+--------+----+--------+| 1 | 张三 | 1 | java || 1 | 张三 | 2 | python || 1 | 张三 | 3 | php || 2 | 李四 | 2 | python || 2 | 李四 | 3 | php || 3 | 王五 | 4 | C |+----+--------+----+--------+6 rows in set (0.03 sec)mysql> explain select s.*, c.* from student s, course c,student_course sc where s.id=sc.student_id and c.id = sc.course_id;+----+-------------+-------+------------+--------+----------------------------+---------+---------+-------------------------+------+----------+--------------------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+-------+------------+--------+----------------------------+---------+---------+-------------------------+------+----------+--------------------------------------------+| 1 | SIMPLE | s | NULL | ALL | PRIMARY | NULL | NULL | NULL | 4 | 100.00 | NULL || 1 | SIMPLE | sc | NULL | ALL | fk_course_id,fk_student_id | NULL | NULL | NULL | 6 | 33.33 | Using where; Using join buffer (hash join) || 1 | SIMPLE | c | NULL | eq_ref | PRIMARY | PRIMARY | 4 | mysql_test.sc.course_id | 1 | 100.00 | NULL |+----+-------------+-------+------------+--------+----------------------------+---------+---------+-------------------------+------+----------+--------------------------------------------+3 rows in set, 1 warning (0.00 sec)
说明1:这一个select语句中,涉及到了三个表,所以有三条执行记录。
说明2:虽然搜索的顺序是student,course,student_course,但是执行顺序是student,student_course,course,因为两个表是没有关系的,需要依靠第三张关系表维系
说明3:这是一个三个都是相同id的案例
mysql> select * from student where id in(select student_id from student_course where course_id = (select id from course where name = "python"));+----+--------+| id | name |+----+--------+| 1 | 张三 || 2 | 李四 |+----+--------+2 rows in set (0.00 sec)mysql> explain select * from student where id in(select student_id from student_course where course_id = (select id from course where name = "python"));+----+--------------+----------------+------------+--------+----------------------------+--------------+---------+------------------------+------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+--------------+----------------+------------+--------+----------------------------+--------------+---------+------------------------+------+----------+-------------+| 1 | PRIMARY | <subquery2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | 100.00 | NULL || 1 | PRIMARY | student | NULL | eq_ref | PRIMARY | PRIMARY | 4 | <subquery2>.student_id | 1 | 100.00 | NULL || 2 | MATERIALIZED | student_course | NULL | ref | fk_course_id,fk_student_id | fk_course_id | 4 | const | 2 | 100.00 | Using where || 3 | SUBQUERY | course | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where |+----+--------------+----------------+------------+--------+----------------------------+--------------+---------+------------------------+------+----------+-------------+4 rows in set, 1 warning (0.00 sec)
说明1:id值越大,越先被执行,所以这个查询,先执行course表的查询,在执行student_course表,最后执行student表
参数select_type:表示select的类型,常见的取值有,SIMPLE、PRIMARY、UNION、SUBQUERY
参数type:表示连接的类型,性能由好到差的链接类型为NULL、system、const、eq_ref、ref、range、index、all;
- 当查询语句中不使用任何表,则查询类型为最优的,但是却在实际工作中,很难做到,不查询表,不然查询的意义是什么呢。
- 当查询系统表的时候,type会为system,所以一般系统表查询比较快
- 当查询id主键或者唯一索引的时候,会出现const类型
- 当查询使用非唯一索引的时候,会出现ref
- 当全表查询的时候会出现all
参数possible_key:可能的索引,一个或者多个
参数key:是实际用到的索引,如果为NULL,则表示没有使用索引
参数key_len:表示索引中使用的字节数,该值为索引字段最大可能长度,并非实际使用长度,在不损失精确性的前提下,长度越短越好。
参数rows:MySQL认为必须要执行的查询的行数,在InnoDB引擎中,是一个估计值,可能并不总是准确的
参数filtered:表示返回结果的行数占需要读取行数的百分比,filtered的值越大越好
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