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：

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

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

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)