Mysql高级2-SQL性能分析
一、SQL执行频率MySQL客户端 连接成功后，通过show 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 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_idmysql> 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
来源:https://www.cnblogs.com/Se7eN-HOU/p/17580630.html
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