mysql的sql性能分析器_MySQL

bitsCN.com author：skate
time：2012/02/17
 
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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