mysql的sql性能分析器

MySQL 的SQL性能分析器主要用途是显示SQL执行的整个过程中各项资源的使用情况。分析器可以更好的展示
出不良SQL的性能问题所在。

mysql sql profile的使用方法

1.开启mysql sql profile

检查mysql sql profile是否启用
mysql> select @@profiling;
+-------------+
| @@profiling |
+-------------+
|           0 | 
+-------------+
1 row in set (0.01 sec)

默认情况下 profiling 的值为0表示 MySQL SQL Profiler处于OFF状态，如果开启SQL性能分析器后， profiling 的值将为1.

mysql> set profiling=1;
Query OK, 0 rows affected (0.03 sec)

mysql> select @@profiling;
+-------------+
| @@profiling |
+-------------+
|           1 | 
+-------------+
1 row in set (0.01 sec)

上面可以看到profiling已经变为1了，但是这个是session级别的，系统是不支持的。如下测试

退出mysql
mysql> quit
Bye
[root@localhost ~]# mysql -uroot -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.0.45-log Source distribution

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

查看profiling的值
mysql> select @@profiling;
+-------------+
| @@profiling |
+-------------+
|           0 | 
+-------------+
1 row in set (0.01 sec)

发现已经变为默认值0了，那如果设置系统级会如何呢？

mysql> set global profiling=1;
ERROR 1228 (HY000): Variable 'profiling' is a SESSION variable and can't be used with SET GLOBAL
mysql>

看到这里报错了。所以mysql sql profile是session级别的。

2. 举个例如，看如何使用

mysql> create table t5 as select * from t1;
ERROR 1046 (3D000): No database selected
mysql> use backup;
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> create table t5 as select * from t1;
Query OK, 2 rows affected (0.06 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select count(*) from t5;
+----------+
| count(*) |
+----------+
|        2 | 
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from t5;
+----------+
| count(*) |
+----------+
|        2 | 
+----------+
1 row in set (0.00 sec)

mysql> show profiles;
+----------+------------+-------------------------------------+
| Query_ID | Duration   | Query                               |
+----------+------------+-------------------------------------+
|        1 | 0.00382400 | select @@profiling                  | 
|        2 | 0.00268500 | create table t5 as select * from t1 | 
|        3 | 0.00017200 | SELECT DATABASE()                   | 
|        4 | 0.01985400 | show databases                      | 
|        5 | 0.00018900 | show tables                         | 
|        6 | 0.06225200 | create table t5 as select * from t1 | 
|        7 | 0.00368800 | select count(*) from t5             | 
|        8 | 0.00322200 | select count(*) from t5             | 
+----------+------------+-------------------------------------+
8 rows in set (0.01 sec)

mysql> 
mysql> show profile for query 7;
+--------------------+----------+
| Status             | Duration |
+--------------------+----------+
| (initialization)   | 0.000414 | 
| Opening tables     | 0.000599 | 
| System lock        | 0.000254 | 
| Table lock         | 0.000175 | 
| init               | 0.000052 | 
| optimizing         | 0.00001  | 
| executing          | 0.002107 | 
| end                | 0.000042 | 
| query end          | 0.000005 | 
| freeing items      | 0.000014 | 
| closing tables     | 0.000011 | 
| logging slow query | 0.000005 | 
+--------------------+----------+
12 rows in set (0.03 sec)

mysql> show profile for query 8;
+--------------------+----------+
| Status             | Duration |
+--------------------+----------+
| (initialization)   | 0.000064 | 
| Opening tables     | 0.000018 | 
| System lock        | 0.00001  | 
| Table lock         | 0.000013 | 
| init               | 0.00002  | 
| optimizing         | 0.00001  | 
| executing          | 0.002589 | 
| end                | 0.000459 | 
| query end          | 0.000007 | 
| freeing items      | 0.000015 | 
| closing tables     | 0.000012 | 
| logging slow query | 0.000005 | 
+--------------------+----------+
12 rows in set (0.00 sec)

mysql> select sum(format(duration,6)) as duration from information_schema.profiling where query_id=7;
+----------+
| duration |
+----------+
| 0.003688 | 
+----------+
1 row in set (0.02 sec)

mysql> select sum(format(duration,6)) as duration from information_schema.profiling where query_id=8;
+----------+
| duration |
+----------+
| 0.003222 | 
+----------+
1 row in set (0.00 sec)

mysql>

从如上的信息可以看出这两个sql的profile统计信息里，前4项差别比较大，这是两个sql主要区别，第二次查询有很多

缓存了了。SQL 性能分析器可以帮助我们对一些比较难以确定性能问题的 SQL 进行诊断，找出问题根源。

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