如何使用SQL Profiler 性能分析器

http://blog.csdn.net/ylqmf/article/details/6541625

ysql 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。

下面我们举例介绍一下 MySQL SQL Profiler 的使用方法：

首先，开启 MySQL SQL Profiler

mysql> SELECT @@profiling;

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

| @@profiling |

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

| 0 |

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

1 row in set (0.00 sec)

mysql> SET profiling = 1;

Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @@profiling;

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

| @@profiling |

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

| 1 |

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

1 row in set (0.00 sec)

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

通过 sql 性能分析器，我们来对比一下 下列语句前后 2 次执行过程的差异，对我们了解 sql 的详细执行过程是非常有帮助的。

mysql> create table t_engines select * from t_engines1;

Query OK, 57344 rows affected (0.10 sec)

Records: 57344 Duplicates: 0 Warnings: 0

mysql> select count(*) from t_engines;

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

| count(*) |

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

| 57344 |

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

1 row in set (0.00 sec)

mysql> select count(*) from t_engines;

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

| count(*) |

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

| 57344 |

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

1 row in set (0.00 sec)

mysql> SHOW PROFILES;

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

| Query_ID | Duration | Query |

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

| 26 | 0.10213775 | create table t_engines select * from t_engines1 |

| 27 | 0.00032775 | select count(*) from t_engines |

| 28 | 0.00003850 | select count(*) from t_engines |

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

15 rows in set (0.01 sec)

mysql> SHOW PROFILE FOR QUERY 27;

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

| Status | Duration |

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

| (initialization) | 0.00000425 |

| checking query cache for query | 0.00004050 |

| checking permissions | 0.00001050 |

| Opening tables | 0.00018250 |

| System lock | 0.00000450 |

| Table lock | 0.00001775 |

| init | 0.00001075 |

| optimizing | 0.00000550 |

| executing | 0.00002775 |

| end | 0.00000450 |

| query end | 0.00000325 |

| storing result in query cache | 0.00000400 |

| freeing items | 0.00000400 |

| closing tables | 0.00000500 |

| logging slow query | 0.00000300 |

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

15 rows in set (0.00 sec)

mysql> SHOW PROFILE FOR QUERY 28;

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

| Status | Duration |

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

| (initialization) | 0.00000350 |

| checking query cache for query | 0.00000750 |

| checking privileges on cached query | 0.00000500 |

| checking permissions | 0.00000525 |

| sending cached result to client | 0.00001275 |

| logging slow query | 0.00000450 |

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

6 rows in set (0.00 sec)

mysql> SELECT sum( FORMAT(DURATION, 6)) AS DURATION FROM INFORMATION_SCHEMA.PROFILING WHERE QUERY_ID =27 ORDER BY SEQ;

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

| DURATION |

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

| 0.000326 |

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

1 row in set (0.00 sec)

mysql> SELECT sum( FORMAT(DURATION, 6)) AS DURATION FROM INFORMATION_SCHEMA.PROFILING WHERE QUERY_ID =28 ORDER BY SEQ;

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

| DURATION |

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

| 0.000039 |

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

1 row in set (0.00 sec)

mysql>

从上面的例子中我们可以清晰的看出 2 次执行 count 语句的差别， SHOW PROFILE FOR QUERY 27 展现的是第一次 count 统计的执行过程，包含了 Opening tables 、 Table lock 等操作 。而 SHOW PROFILE FOR QUERY 28 展示了第二次 count 统计的执行过程 , 第二次 count 直接从查询缓存中返回 count 统计结果，通过对比 2 次统计的总执行时间发现，缓存读的速度接近物理读的 10 倍。通过使用 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。