Wednesday, September 8, 2010

MySQL性能优化5 - MySQL SQL Profiler 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。

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:
[cc lang="mysql"]
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)[/cc]

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。
[cc lang="mysql"]
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)
[/cc]
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, [cci lang="mysql"]SHOW PROFILE FOR QUERY 27 [/cci]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 [cci lang="mysql"]SHOW PROFILE FOR QUERY 28[/cci] 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。

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