如何使用SQL Profiler 性能分析器

轉自:http://blog.chinaunix.net/space.php?uid=16362696&do=blog&id=2746789

mysql 的 sql 性能分析器主要用途是顯示 sql 執行的整個過程當中各項資源的使用狀況。分析器能夠更好的展現出不良 SQL 的性能問題所在。

下面咱們舉例介紹一下 MySQL SQL Profiler 的使用方法：

• 首先，開啓 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)

默認狀況下 profiling 的值爲 0 表示 MySQL SQL Profiler 處於 OFF 狀態，開啓 SQL 性能分析器後 profiling 的值爲 1.

• 經過 sql 性能分析器，咱們來對比一下 下列語句先後 2 次執行過程的差別，對咱們瞭解 sql 的詳細執行過程是很是有幫助的。

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>

從上面的例子中咱們能夠清晰的看出 2 次執行 count 語句的差異， SHOW PROFILE FOR QUERY 27 展示的是第一次 count 統計的執行過程，包含了 Opening tables 、 Table lock 等操做 。而 SHOW PROFILE FOR QUERY 28 展現了第二次 count 統計的執行過程 , 第二次 count 直接從查詢緩存中返回 count 統計結果，經過對比 2 次統計的總執行時間發現，緩存讀的速度接近物理讀的 10 倍。經過使用 SQL 性能分析器能夠幫助咱們對一些比較難以肯定性能問題的 SQL 進行診斷，找出問題根源。

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能夠使用explain查看查詢命令的執行狀況.是否使用index 等.