explain是从mysql怎样解析执行sql的角度分析sql优劣。profiling是从sql执行时资源使用情况的角度来分析sql。

分析SQL执行带来的开销是优化SQL的重要手段。在MySQL数据库中，可以通过配置profiling参数来启用SQL剖析。该参数可以在全局和session级别来设置。对于全局级别则作用于整个MySQL实例，而session级别紧影响当前session。该参数开启后，后续执行的SQL语句都将记录其资源开销，诸如IO，上下文切换，CPU，Memory等等。根据这些开销进一步分析当前SQL瓶颈从而进行优化与调整。本文描述了如何使用MySQL profile，不涉及具体的样例分析。

1、有关profile的描述

查看profiling系统变量

mysql> show variables like '%profil%';

+------------------------+-------+

| Variable_name | Value |

+------------------------+-------+

| have_profiling | YES | #只读变量,用于控制是否由系统变量开启或禁用profiling;

| profiling | OFF | #开启或关闭SQL语句剖析功能;

| profiling_history_size | 15 | #设置保留profiling的数目,缺省为15,范围为0至100,为0时将禁用profiling;

+------------------------+-------+

3 rows in set (0.00 sec)

获取profile的帮助

mysql> help profile;

Name: 'SHOW PROFILE'

Description:

Syntax:

SHOW PROFILE [type [, type] ... ]

[FOR QUERY n]

[LIMIT row_count [OFFSET offset]]

type:

ALL #显示所有的开销信息;

| BLOCK IO #显示块IO相关开销;

| CONTEXT SWITCHES #上下文切换相关开销;

| CPU #显示CPU相关开销信息;

| IPC #显示发送和接收相关开销信息;

| MEMORY #显示内存相关开销信息;

| PAGE FAULTS #显示页面错误相关开销信息;

| SOURCE #显示和Source_function,Source_file,Source_line相关的开销信息;

| SWAPS #显示交换次数相关开销的信息;

2、开启porfiling

启用session级别的profiling

mysql> set profiling=1;

Query OK, 0 rows affected, 1 warning (0.00 sec)

验证修改后的结果

mysql> show variables like '%profil%';

+------------------------+-------+

| Variable_name | Value |

+------------------------+-------+

| have_profiling | YES |

| profiling | ON |

| profiling_history_size | 15 |

+------------------------+-------+

3 rows in set (0.00 sec)

发布SQL查询

mysql> select * from zabbix.history_uint where clock<1488466329 limit 1;

查看当前session所有已产生的profile

mysql> show profiles;

+----------+------------+------------------------------------------------------------------+

| Query_ID | Duration | Query |

+----------+------------+------------------------------------------------------------------+

| 1 | 0.00044625 | show variables like '%profil%' |

| 2 | 6.43230200 | select * from zabbix.history_uint where clock<1488466329 limit 1 |

+----------+------------+------------------------------------------------------------------+

2 rows in set, 1 warning (0.00 sec)

我们看到有2个warning，之前一个，现在一个

mysql> show warnings;

+---------+------+--------------------------------------------------------------------------------------------------------------+

| Level | Code | Message |

+---------+------+--------------------------------------------------------------------------------------------------------------+

| Warning | 1287 | 'SHOW PROFILES' is deprecated and will be removed in a future release. Please use Performance Schema instead |

+---------+------+--------------------------------------------------------------------------------------------------------------+

1 row in set (0.00 sec)

告警是说SHOW PROFILES命令将来会被Performance Schema替换掉。

3、获取SQL语句的开销信息

开启profiling后，我们可以通过show profile等方式查看，其实这些开销信息被记录到information_schema.profiling表中。注show profile之类的语句不会被profiling，即自身不会产生Profiling。

我们下面的这个show profile查看的是show warnings产生的相应开销。

mysq> show profile;

+----------------+----------+

| Status | Duration |

+----------------+----------+

| starting | 0.000141 |

| query end | 0.000058 |

| closing tables | 0.000014 |

| freeing items | 0.001802 |

| cleaning up | 0.000272 |

+----------------+----------+

如下面的查询show warnings被添加到profiles

mysql> show profiles;

+----------+------------+------------------------------------------------------------------+

| Query_ID | Duration | Query |

+----------+------------+------------------------------------------------------------------+

| 1 | 0.00041150 | show variables like '%profil%' |

| 2 | 6.41118075 | select * from zabbix.history_uint where clock<1488466329 limit 1 |

| 3 | 0.00003900 | show warnings |

+----------+------------+------------------------------------------------------------------+

3 rows in set, 1 warning (0.00 sec)

获取指定查询的开销(Druation表示持续时间)

mysql> show profile for query 2;

+----------------------+----------+

| Status | Duration |

+----------------------+----------+

| starting | 0.000043 |

| checking permissions | 0.000004 |

| Opening tables | 0.000014 |

| init | 0.000019 |

| System lock | 0.000004 |

| optimizing | 0.000009 |

| statistics | 0.000062 |

| preparing | 0.000012 |

| executing | 0.000002 |

| Sending data | 6.410967 |

| end | 0.000009 |

| query end | 0.000005 |

| closing tables | 0.000010 |

| freeing items | 0.000011 |

| cleaning up | 0.000011 |

+----------------------+----------+

15 rows in set, 1 warning (0.00 sec)

Sending data：这个状态的名称很具有误导性，所谓的“Sending data”并不是单纯的发送数据，而是包括“收集 + 发送 数据”。

query end：表示语句执行完毕了，但是还有一些后续工作没做完时的状态。

freeing items：释放查询缓存里面的空间，如果是DML操作，所以相应的缓存里的记录就无效了，所以需要有这一步做处理。

查看所有开销

mysql> show profile all for query 2 ;

查看特定部分的开销，如下为CPU部分的开销

mysql> show profile cpu for query 2 ;

如下为MEMORY部分的开销

mysql> show profile memory for query 2 ;

同时查看不同资源开销

mysql> show profile block io,cpu for query 2;

4、INFORMATION_SCHEMA.PROFILING

上面已经看到了，show profile命令即将移除，所以可以直接去information_schema.profiling表查看，灵活度更大，其表结构信息如下：

mysql> desc information_schema.profiling;

+---------------------+--------------+------+-----+----------+-------+

| Field | Type | Null | Key | Default | Extra |

+---------------------+--------------+------+-----+----------+-------+

| QUERY_ID | int(20) | NO | | 0 | |

| SEQ | int(20) | NO | | 0 | |

| STATE | varchar(30) | NO | | | |

| DURATION | decimal(9,6) | NO | | 0.000000 | |

| CPU_USER | decimal(9,6) | YES | | NULL | |

| CPU_SYSTEM | decimal(9,6) | YES | | NULL | |

| CONTEXT_VOLUNTARY | int(20) | YES | | NULL | |

| CONTEXT_INVOLUNTARY | int(20) | YES | | NULL | |

| BLOCK_OPS_IN | int(20) | YES | | NULL | |

| BLOCK_OPS_OUT | int(20) | YES | | NULL | |

| MESSAGES_SENT | int(20) | YES | | NULL | |

| MESSAGES_RECEIVED | int(20) | YES | | NULL | |

| PAGE_FAULTS_MAJOR | int(20) | YES | | NULL | |

| PAGE_FAULTS_MINOR | int(20) | YES | | NULL | |

| SWAPS | int(20) | YES | | NULL | |

| SOURCE_FUNCTION | varchar(30) | YES | | NULL | |

| SOURCE_FILE | varchar(20) | YES | | NULL | |

| SOURCE_LINE | int(20) | YES | | NULL | |

+---------------------+--------------+------+-----+----------+-------+

18 rows in set (0.00 sec)

下面的SQL语句用于查询query_id为2的SQL开销，且按最大耗用时间倒序排列

mysql> set @query_id=2;

SELECT STATE, SUM(DURATION) AS Total_R,

ROUND(

100 * SUM(DURATION) /

(SELECT SUM(DURATION)

FROM INFORMATION_SCHEMA.PROFILING

WHERE QUERY_ID = @query_id

), 2) AS Pct_R,

COUNT(*) AS Calls,

SUM(DURATION) / COUNT(*) AS "R/Call"

FROM INFORMATION_SCHEMA.PROFILING

WHERE QUERY_ID = @query_id

GROUP BY STATE

ORDER BY Total_R DESC;

+----------------------+----------+--------+-------+--------------+

| STATE | Total_R | Pct_R | Calls | R/Call |

+----------------------+----------+--------+-------+--------------+

| Sending data | 6.410967 | 100.00 | 1 | 6.4109670000 |

| statistics | 0.000062 | 0.00 | 1 | 0.0000620000 |

| starting | 0.000043 | 0.00 | 1 | 0.0000430000 |

| init | 0.000019 | 0.00 | 1 | 0.0000190000 |

| Opening tables | 0.000014 | 0.00 | 1 | 0.0000140000 |

| preparing | 0.000012 | 0.00 | 1 | 0.0000120000 |

| cleaning up | 0.000011 | 0.00 | 1 | 0.0000110000 |

| freeing items | 0.000011 | 0.00 | 1 | 0.0000110000 |

| closing tables | 0.000010 | 0.00 | 1 | 0.0000100000 |

| optimizing | 0.000009 | 0.00 | 1 | 0.0000090000 |

| end | 0.000009 | 0.00 | 1 | 0.0000090000 |

| query end | 0.000005 | 0.00 | 1 | 0.0000050000 |

| System lock | 0.000004 | 0.00 | 1 | 0.0000040000 |

| checking permissions | 0.000004 | 0.00 | 1 | 0.0000040000 |

| executing | 0.000002 | 0.00 | 1 | 0.0000020000 |

+----------------------+----------+--------+-------+--------------+

15 rows in set (0.01 sec)

停止profile,可以设置profiling参数，或者在session退出之后,profiling会被自动关闭。

从以上可以看出，profiling即是将mysql处理sql的各个阶段的资源消耗查询统计出来。