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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