MYSQL查询优化：show profile

为避免之前已经把 SQL 存放在 QCACHE 中， 建议在执行 SQL 时， 强制 SELECT 语句不进行 QCACHE 检测。这样可以提交分析的准确性

mysql> select sql_no_cache  count(*) from system_user;

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Query Profiler是MYSQL自带的一种query诊断分析工具，通过它可以分析出一条SQL语句的性能瓶颈在什么地方。通常我们是使用的explain,以及slow query log都无法做到精确分析，但是Query Profiler却可以定位出一条SQL语句执行的各种资源消耗情况，比如CPU，IO等，以及该SQL执行所耗费的时间等。不过该工具只有在MYSQL 5.0.37以及以上版本中才有实现。
默认的情况下，MYSQL的该功能没有打开，需要自己手动启动。可以通过如下方法查看当前mysql服务器是否开启了该功能。

MySQL查找SQL耗时瓶颈 SHOW profiles

1、不过版本要在5.0.37之后。(SHOW PROFILES and SHOW PROFILE were added in MySQL 5.0.37.)

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 sql_no_cache 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

2、变量profiling是用户变量，每次都得重新启用。
以下是我做的一些实验。数据很明显，就不多解释了。

mysql> use test
Database changed
mysql> set profiling=1;
Query OK, 0 rows affected (0.00 sec)

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| bag_item |
| bag_user |
| score |
| t |
+----------------+
4 rows in set (0.03 sec)

mysql> select count(*) from t;
+----------+
| count(*) |
+----------+
| 2097152 |
+----------+
1 row in set (0.74 sec)

mysql> show profiles;
+----------+------------+------------------------+
| Query_ID | Duration   | Query                  |
+----------+------------+------------------------+
|        1 | 0.02717000 | show tables            |
|        2 | 0.74770100 | select count(*) from t |
+----------+------------+------------------------+
2 rows in set (0.00 sec)

mysql> show profile for query 2;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| (initialization)               | 0.000004 |
| checking query cache for query | 0.000044 |
| Opening tables                 | 0.000012 |
| System lock                    | 0.000017 |
| Table lock                     | 0.00003  |
| init                           | 0.000013 |
| optimizing                     | 0.000008 |
| statistics                     | 0.000013 |
| preparing                      | 0.000011 |
| executing                      | 0.000006 |
| Sending data                   | 0.747313 |
| end                            | 0.000014 |
| query end                      | 0.000006 |
| storing result in query cache  | 0.000006 |
| freeing items                  | 0.000012 |
| closing tables                 | 0.000009 |
| logging slow query             | 0.000183 |
+--------------------------------+----------+
17 rows in set (0.00 sec)

mysql> show profile block io,cpu for query 2;
+--------------------------------+----------+----------+------------+--------------+---------------+
| Status                         | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+--------------------------------+----------+----------+------------+--------------+---------------+
| (initialization)               | 0.000004 | 0        | 0          |            0 |             0 |
| checking query cache for query | 0.000044 | 0        | 0          |            0 |             0 |
| Opening tables                 | 0.000012 | 0        | 0          |            0 |             0 |
| System lock                    | 0.000017 | 0        | 0          |            0 |             0 |
| Table lock                     | 0.00003  | 0        | 0          |            0 |             0 |
| init                           | 0.000013 | 0        | 0          |            0 |             0 |
| optimizing                     | 0.000008 | 0        | 0          |            0 |             0 |
| statistics                     | 0.000013 | 0        | 0          |            0 |             0 |
| preparing                      | 0.000011 | 0        | 0          |            0 |             0 |
| executing                      | 0.000006 | 0        | 0          |            0 |             0 |
| Sending data                   | 0.747313 | 0.746887 | 0          |            0 |             0 |
| end                            | 0.000014 | 0        | 0          |            0 |             0 |
| query end                      | 0.000006 | 0        | 0          |            0 |             0 |
| storing result in query cache  | 0.000006 | 0        | 0          |            0 |             0 |
| freeing items                  | 0.000012 | 0        | 0          |            0 |             0 |
| closing tables                 | 0.000009 | 0        | 0          |            0 |             0 |
| logging slow query             | 0.000183 | 0        | 0          |            0 |             0 |
+--------------------------------+----------+----------+------------+--------------+---------------+
17 rows in set (0.00 sec)

mysql> insert into t(username) select username from t;
Query OK, 2097152 rows affected (34.17 sec)
Records: 2097152  Duplicates: 0  Warnings: 0

mysql> show profiles;
+----------+-------------+------------------------------------------------+
| Query_ID | Duration    | Query                                          |
+----------+-------------+------------------------------------------------+
|        1 |  0.02717000 | show tables                                    |
|        2 |  0.74770100 | select count(*) from t                         |
|        3 |  0.00004200 | show prifile for query 2                       |
|        4 | 34.30410100 | insert into t(username) select username from t |
+----------+-------------+------------------------------------------------+
4 rows in set (0.00 sec)

mysql> show profile cpu,block io,memory,swaps for query 4;

mysql> select count(*) from t;
+----------+
| count(*) |
+----------+
|  4194304 |
+----------+
1 row in set (1.51 sec)

mysql> show profiles;
+----------+-------------+------------------------------------------------+
| Query_ID | Duration    | Query                                          |
+----------+-------------+------------------------------------------------+
|        1 |  0.02717000 | show tables                                    |
|        2 |  0.74770100 | select count(*) from t                         |
|        3 |  0.00004200 | show prifile for query 2                       |
|        4 | 34.30410100 | insert into t(username) select username from t |
|        5 |  1.50563800 | select count(*) from t                         |
+----------+-------------+------------------------------------------------+
5 rows in set (0.00 sec)

mysql> show profile cpu,block io,memory,swaps,context switches,source for query 5;
……
mysql> update t set username = 'waill';
Query OK, 4194304 rows affected (44.82 sec)
Rows matched: 4194304  Changed: 4194304  Warnings: 0

mysql> show profiles;
+----------+-------------+------------------------------------------------+
| Query_ID | Duration    | Query                                          |
+----------+-------------+------------------------------------------------+
|        1 |  0.02717000 | show tables                                    |
|        2 |  0.74770100 | select count(*) from t                         |
|        3 |  0.00004200 | show prifile for query 2                       |
|        4 | 34.30410100 | insert into t(username) select username from t |
|        5 |  1.50563800 | select count(*) from t                         |
|        6 | 44.82054700 | update t set username = 'waill'                |
+----------+-------------+------------------------------------------------+
6 rows in set (0.00 sec)

mysql> show profile cpu,block io,memory,swaps,context switches,source for query 6;
mysql> show profile all for query 6;

测试完毕以后 ，关闭参数：
mysql> set profiling=0