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的访问频次
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
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。
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。
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)
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)
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)
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)
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
explain/desc select 字段列表 from 表名 where 条件；
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)
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;
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