sql优化之：mysql的explain与profile

在mysql查询性能分析中最常用的就是explain了，profile查看一些具体的性能也是不错的
1. profile
我们可以先使用
SELECT @@profiling;
来查看是否已经启用profile，如果profilng值为0，可以通过
SET profiling = 1;
来启用。启用profiling之后，我们执行一条查询语句，比如：
select count(*) from roi_summary;
然后show profiles查看如下：
+----------+------------+----------------------------------+
| Query_ID | Duration       | Query                            |
+----------+------------+----------------------------------+
|        1       | 0.00021500 | select @@profiling               |
|        2       | 0.05522700 | select count(*) from roi_summary |
+----------+------------+----------------------------------+
2 rows in set (0.00 sec)
其中ID为5的语句是刚执行的查询语句，这时候我们执行show profile for query 2来查看这条语句的执行过程如下;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000021 |
| checking query cache for query | 0.000045 |
| checking permissions           | 0.000007 |
| Opening tables                 | 0.000011 |
| System lock                    | 0.000004 |
| Table lock                     | 0.000040 |
| init                           | 0.000012 |
| optimizing                     | 0.000005 |
| statistics                     | 0.000010 |
| preparing                      | 0.000010 |
| executing                      | 0.000005 |
| Sending data                   | 0.055021 |
| end                            | 0.000007 |
| end                            | 0.000004 |
| query end                      | 0.000003 |
| storing result in query cache | 0.000004 |
| freeing items                  | 0.000008 |
| closing tables                 | 0.000005 |
| logging slow query             | 0.000002 |
| cleaning up                    | 0.000003 |
+--------------------------------+----------+
20 rows in set (0.00 sec)
可以看出此条查询语句的执行过程及执行时间，总的时间约为0.05s。
这时候我们再执行一次
select count(*) from roi_summary;
show profiles;
+----------+------------+----------------------------------+
| Query_ID | Duration   | Query                            |
+----------+------------+----------------------------------+
|        1 | 0.00021500 | select @@profiling               |
|        2 | 0.05522700 | select count(*) from roi_summary |
|        3 | 0.00006000 | select count(*) from roi_summary |
+----------+------------+----------------------------------+
然后执行show profile for query 3来查看本条语句的执行过程
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000016 |
| checking query cache for query | 0.000007 |
| checking privileges on cached | 0.000004 |
| checking permissions           | 0.000005 |
| sending cached result to clien | 0.000022 |
| logging slow query             | 0.000003 |
| cleaning up                    | 0.000003 |
+--------------------------------+----------+
可以看出此次第二次查询因为前一次的查询生成了cache，所以这次无需从数据库文件中再次读取数据而是直接从缓存中读取，结果查询时间比第一次快了N倍。
2. explain
至于EXPLAIN 官方文档讲解的相当详细了，
地址：dev.mysql.com/doc/refman/5.1/zh/optimization.html#explain

来自：nonb.cn