mysql的sql性能分析器

超、凢脫俗 2022-06-15 05:13 147阅读 0赞

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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