总在sending data阶段耗时过多的sql语句，应该如何优化？

Question

我的一条sql语句，索引都已经建立好了，没有任何的filesort,和临时表的操作，explain后显示的性能很不错！
一般来说是很快的，但是经过show profile cpu, block io for query n;的操作发现，有时候会在sending data耗时过多！
而且同一条sql语句不同时候在sending data这个阶段耗费的时间差距非常大，从不到一秒到十几秒不等，测试的时候并没有其它访问阻碍！这样的情况应该从何处入手呢？

Answer 1

嗯，不过我这条sql确实没有 *

Answer 2

acmain版的意思是说，sending data主要受到系统IO的影响？

Answer 3

| executing          | 0.000003 |
| Sending data       | 0.000167 |Sending data 这一行的 0.000167 其实是 executing 到 Sending data 所花的时间。

Answer 4

不是太明白，那 Sending data本身到底处于什么操作中呢？

Answer 5

这是我sending data的一个列子，不知该如何看！mysql> show profile cpu, block io for query 2;
+--------------------------------+-----------+-----------+------------+--------------+---------------+
| Status                         | Duration  | CPU_user  | CPU_system | Block_ops_in | Block_ops_out |
+--------------------------------+-----------+-----------+------------+--------------+---------------+
| starting                       |  0.000017 |  0.000000 |   0.000000 |            0 |             0 | 
| checking query cache for query |  0.000086 |  0.000000 |   0.000000 |            0 |             0 | 
| Opening tables                 |  0.000017 |  0.000000 |   0.000000 |            0 |             0 | 
| System lock                    |  0.000013 |  0.000000 |   0.000000 |            0 |             0 | 
| Table lock                     |  0.000012 |  0.000000 |   0.000000 |            0 |             0 | 
| init                           |  0.000030 |  0.000000 |   0.000000 |            0 |             0 | 
| optimizing                     |  0.000031 |  0.000000 |   0.000000 |            0 |             0 | 
| statistics                     |  0.000289 |  0.000000 |   0.000000 |            0 |             0 | 
| preparing                      |  0.000023 |  0.000000 |   0.000000 |            0 |             0 | 
| executing                      |  0.000004 |  0.000000 |   0.000000 |            0 |             0 | 
| Sorting result                 |  0.000004 |  0.000000 |   0.000000 |            0 |             0 | 
| Sending data                   | 12.169379 | 18.651165 |   4.117374 |            0 |             0 | 
| end                            |  0.000010 |  0.000000 |   0.000000 |            0 |             0 | 
| query end                      |  0.000004 |  0.000000 |   0.000000 |            0 |             0 | 
| freeing items                  |  0.000605 |  0.001000 |   0.000000 |            0 |             0 | 
| logging slow query             |  0.000004 |  0.000000 |   0.000000 |            0 |             0 | 
| logging slow query             |  0.000039 |  0.000000 |   0.000000 |            0 |             0 | 
| cleaning up                    |  0.000004 |  0.000000 |   0.000000 |            0 |             0 | 
+--------------------------------+-----------+-----------+------------+--------------+---------------+