Mysql Profiling的使用

profiling是个很好用的mysql性能分析工具，今儿就来试验下profiling的功能。感谢 有爱玫瑰的博文：

 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。

    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)

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。