sending data->索引问题

最近，有两次Tomcat 响应很慢，直到宕机。期间登录mysql server ，使用show processlist均能看到大量的sending data 状态。查看官方的关于sending data说明：

• Sending data

  The thread is processing rows for a statement and also is sending data to the client.

意思是正在将查询结果往客户端发送。


开始一直以为是max_allow_packet的值太小，并且认为程序有问题导致tomcat响应很慢。mysql 将查询的结果发送给前端tomcat.而此时tomcat因程序问题响应很慢，所以mysql将查询到的结果一直处理Sending data状态。但是让我不明白的是我前端有两个同样的tomcat，为什么同时都有问题。

后来仔细分析了两次出现大量Sending data的sql语句。发现大部分都是针对一个表上的查询。而且where 条件的那个字段值只有几种，大部分都是相同的值。这样在上面建索引根本没有任何的效果。同时用profile验证了这个sql查询每个阶段所需要的时间。发现在Sending data 上花费的时间最长。expalin 出来的结果显示共扫描了161172行。

mysql> explain select P.ID_ FROM NEWSFEEDDIST P WHERE RELATIONSHIP_ = 1 and CREATED_ >= DATE_SUB(NOW(),INTERVAL 18 DAY) AND CREATED_ < DATE_SUB(NOW(),INTERVAL 17 DAY);        +----+-------------+-------+------+-------------------------+-------------------------+---------+-------+--------+-------------+
| id | select_type | table | type | possible_keys           | key                     | key_len | ref   | rows   | Extra       |
+----+-------------+-------+------+-------------------------+-------------------------+---------+-------+--------+-------------+
| 1 | SIMPLE      | P     | ref | SNS_NEWSFEEDDIST_REL_IN | SNS_NEWSFEEDDIST_REL_IN | 3       | const | 161172 | Using where |
+----+-------------+-------+------+-------------------------+-------------------------+---------+-------+--------+-------------+

mysql> set profiling =1;

mysql> SELECT COUNT(1), MAX(P.CREATED_) FROM NEWSFEEDDIST P WHERE RELATIONSHIP_ = 1 AND CREATED_ >= DATE_SUB(NOW(),INTERVAL 19 DAY) AND CREATED_ < DATE_SUB(NOW(),INTERVAL 18 DAY);

mysql> show profile for query 1;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.060039 |
| checking query cache for query | 0.000161 |
| Opening tables                 | 0.000034 |
| System lock                    | 0.000019 |
| Table lock                     | 0.000021 |
| init                           | 0.000095 |
| optimizing                     | 0.000035 |
| statistics                     | 0.000157 |
| preparing                      | 0.000040 |
| executing                      | 0.000010 |
| Sending data                   | 1.079902 |
| end                            | 0.000018 |
| query end                      | 0.000005 |
| freeing items                  | 0.000056 |
| logging slow query             | 0.000004 |
| logging slow query             | 0.000053 |
| cleaning up                    | 0.000007 |
+--------------------------------+----------+
17 rows in set (0.00 sec)

既然 RELATIONSHIP_ 字段上的索引无效，那么针对这个sql我在CREATED_字段上创建了一个索引。再次运行
mysql>alter table NEWSFEEDDIST add index(CREATED_);
mysql>SELECT COUNT(1), MAX(P.CREATED_) FROM NEWSFEEDDIST P WHERE RELATIONSHIP_ = 1 AND CREATED_ >= DATE_SUB(NOW(),INTERVAL 19 DAY) AND CREATED_ < DATE_SUB(NOW(),INTERVAL 18 DAY);
mysql>show profile for query 3;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000048 |
| checking query cache for query | 0.000130 |
| Opening tables                 | 0.000028 |
| System lock                    | 0.000011 |
| Table lock                     | 0.000019 |
| init                           | 0.000076 |
| optimizing                     | 0.000031 |
| statistics                     | 0.000183 |
| preparing                      | 0.000034 |
| executing                      | 0.000011 |
| Sending data                   | 0.007954 |
| end                            | 0.000013 |
| query end                      | 0.000009 |
| freeing items                  | 0.000088 |
| logging slow query             | 0.000009 |
| cleaning up                    | 0.000011 |
+--------------------------------+----------+
16 rows in set (0.00 sec)
此时在sending data上小了很多。而且用了CREATED_索引后扫描的行数变成了2888条

mysql>explain select P.ID_ FROM test_NEWSFEEDDIST P WHERE RELATIONSHIP_ = 1 and CREATED_ >= DATE_SUB(NOW(),INTERVAL 18 DAY) AND CREATED_ < DATE_SUB(NOW(),INTERVAL 17 DAY);                                    
+----+-------------+-------+-------+----------------------------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys                    | key      | key_len | ref | rows | Extra       |
+----+-------------+-------+-------+----------------------------------+----------+---------+------+------+-------------+
| 1 | SIMPLE      | P     | range | SNS_NEWSFEEDDIST_REL_IN,CREATED_ | CREATED_ | 9       | NULL | 2888 | Using where |
+----+-------------+-------+-------+----------------------------------+----------+---------+------+------+-------------+


这个效果还是不错的。cool


这里有个关于mysql sending data状态的解释
http://renxijun.blog.sohu.com/82906360.html

 

http://hi.baidu.com/hexie007/item/126939785b86353c714423cd