MySQL SQL Tuning：Profile定位单条QUERY性能瓶颈_MySQL

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MySQL SQL Tuning：Profile定位单条QUERY性能瓶颈

当生了病的query来就诊时，如果身为医生的我们"胡庸医乱用虎狼药"

不仅于事无补，还浪费了大量的人力和时间成本，甚至会拖垮服务器

所以，我们在接受优化一条SQL，第一件事情便是要明白query病在哪里？

是IO？是CPU?只有明白瓶颈在哪里，方可对症下药，也才能药到病除

而MySQL QUERY Profiler是一个使用非常方便的QUERY诊断工具，5.0引入。5.1GA版嵌入

这个工具详细呈现了SQL在整个生命周期的每个动作，这和Oracle开启1046事件类似

我们可以很清晰地明白该SQL是在数据存取还是运算(排序或分组等)上花费得多

那么我们就不会很盲目地看到order by就去tuning sort buffer而忽略sorting result时间是如此之少

Profile语法：

[plain] SHOW PROFILE [type [, type] ... ]      [FOR QUERY n]      [LIMIT row_count [OFFSET offset]]    type:      ALL    | BLOCK IO    | CONTEXT SWITCHES    | CPU    | IPC    | MEMORY    | PAGE FAULTS    | SOURCE    | SWAPS  

注解：

默认输出结果只会展示Status和Duration，我们可以指定type来扩展输出

我比较常用的是外加CPU和BLOCK IO来输出CPU和IO的负载，其实这些已经够了

默认profile是关闭的，通过profiling参数控制，为session级

开启：SET profiling=1

关闭：set profiling=0

查询：select @@profiling

show profiles保存的query条数由参数profiling_history_size控制，默认是15，超过了会把前面的剔掉

[plain] mysql> set profiling=1;    mysql> select @@profiling;  +-------------+  | @@profiling |  +-------------+  |           1 |  +-------------+    mysql> select * from employees.t order by first_name;    mysql> show profiles;  +----------+------------+-----------------------------------------------+  | Query_ID | Duration   | Query                                         |  +----------+------------+-----------------------------------------------+  |        1 | 0.21138800 | show create table employees.t                 |  |        2 | 8.21691600 | select * from employees.t order by first_name |  +----------+------------+-----------------------------------------------+  2 rows in set (0.00 sec)    mysql> show profile cpu,block io for query 2;  +----------------------+----------+----------+------------+--------------+---------------+  | Status               | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |  +----------------------+----------+----------+------------+--------------+---------------+  | starting             | 0.000160 | 0.000000 |   0.000000 |            0 |             0 |  | checking permissions | 0.000026 | 0.000000 |   0.000000 |            0 |             0 |  | Opening tables       | 0.000055 | 0.000000 |   0.000000 |            0 |             0 |  | System lock          | 0.000033 | 0.000000 |   0.000000 |            0 |             0 |  | init                 | 0.000050 | 0.000000 |   0.000000 |            0 |             0 |  | optimizing           | 0.000026 | 0.000000 |   0.000000 |            0 |             0 |  | statistics           | 0.000145 | 0.000000 |   0.000000 |            0 |             0 |  | preparing            | 0.000118 | 0.000000 |   0.000000 |            0 |             0 |  | executing            | 0.000011 | 0.000000 |   0.000000 |            0 |             0 |  | Sorting result       | 2.838465 | 1.396087 |   1.140071 |            0 |             0 |  | Sending data         | 0.928078 | 0.544034 |   0.056003 |            0 |             0 |  | end                  | 0.000026 | 0.000000 |   0.000000 |            0 |             0 |  | query end            | 0.000011 | 0.000000 |   0.000000 |            0 |             0 |  | closing tables       | 0.000021 | 0.000000 |   0.000000 |            0 |             0 |  | freeing items        | 4.449672 | 0.000000 |   0.000000 |            0 |             0 |  | logging slow query   | 0.000014 | 0.000000 |   0.000000 |            0 |             0 |  | cleaning up          | 0.000005 | 0.000000 |   0.000000 |            0 |             0 |  +----------------------+----------+----------+------------+--------------+---------------+  17 rows in set (0.00 sec)  

相同的信息我们还可以从information_schema里面输出，并且我们还可以对时间进行排序

因为Profile默认是按执行顺序排序的，而实际上我们更关心的是花费了多少时间，这才能方便知道哪些开销较大

[plain] mysql> SELECT STATE, FORMAT(DURATION, 6) AS DURATION      -> FROM INFORMATION_SCHEMA.PROFILING      -> WHERE QUERY_ID = 2 ORDER BY DURATION DESC;  +----------------------+----------+  | STATE                | DURATION |  +----------------------+----------+  | freeing items        | 4.449672 |  | Sorting result       | 2.838465 |  | Sending data         | 0.928078 |  | starting             | 0.000160 |  | statistics           | 0.000145 |  | preparing            | 0.000118 |  | Opening tables       | 0.000055 |  | init                 | 0.000050 |  | System lock          | 0.000033 |  | end                  | 0.000026 |  | optimizing           | 0.000026 |  | checking permissions | 0.000026 |  | closing tables       | 0.000021 |  | logging slow query   | 0.000014 |  | query end            | 0.000011 |  | executing            | 0.000011 |  | cleaning up          | 0.000005 |  +----------------------+----------+  17 rows in set (0.00 sec)  

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