关于MYSQL组合索引的使用，官方对下面的例子的说法是可以使用索引：
KEY（key_part1,key_part2,key_part3）
select .... from table where key_part1='xxx' and key_part3='yyy';
从MYSQL的执行计划看，确实也是使用索引;
但在实际的优化过程中，我们只是简单的关注是否使用了这个索引是不够的。
[@more@]我们需要关注的是:
对key_part3这个关键字过滤的时候，是否用到了索引？
下面我们来创建一个例子：
CREATE TABLE `im_message_201001_12` (
`msg_id` bigint(20) NOT NULL default '0',
`time` datetime NOT NULL,
`owner` varchar(64) collate latin1_bin NOT NULL,
`other` varchar(64) collate latin1_bin NOT NULL,
`content` varchar(8000) collate latin1_bin default NULL,
PRIMARY KEY (`msg_id`),
KEY `im_msg_own_oth_tim_ind` (`owner`,`other`,`time`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_bin;
查询语句:
select count(distinct concat('ab',content)) dis ,count(*) all from im_message_201001_12
where
owner='huaniaoyuchong83'
and time between '2010-01-01 00:00:00' and '2010-02-01 00:00:00' ;
我们看到，查询的条件，对索引来说是跳跃的。
这对ORACLE来说并不是难事。SQL优化器会在索引里完成对time字段的过滤。
用HINT： 可以来辅助。
但对MYSQL来说，你可能并不知道，是什么时候对time字段进行过滤的。
当然我们希望是通过索引来过滤TIME字段。这样最后回表的次数就会少一些。
在测试过程中，我们通过观察MYSQL的Innodb_buffer_pool_read_requests（逻辑读）变量的变化，来推测结果。
注意以下查询过程中，条件time的变化，以及变量Innodb_buffer_pool_read_requests的变化
#######测试环境：
OS：RHEL 4.7 X86_64
MYSQL 5.0.51a / 5.1.40
请在开始下面测试前，运行：
select count(distinct concat('c',content)),count(*) from im_message_201001_11 where owner='huaniaoyuchong83' ；
以让所有结果都在CACHE里；
#######开始第一次测试
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 136566076 |
+----------------------------------+-----------+
1 row in set (0.02 sec)
select count(distinct concat('c',content)),count(*) from im_message_201001_11 where owner='huaniaoyuchong83' and time between '2010-01-01 00:00:00' and '2010-02-01 00:00:00' ;
+-------------------------------------+----------+
| count(distinct concat('c',content)) | count(*) |
+-------------------------------------+----------+
| 35644 | 44397 |
+-------------------------------------+----------+
1 row in set (1.40 sec)
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 136742193 |
+----------------------------------+-----------+
1 row in set (0.02 sec)
select 136742193-136566076 ;
+---------------------+
| 136742193-136566076 |
+---------------------+
| 176117 |
+---------------------+
1 row in set (0.00 sec)
#######开始第二次测试
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 136742194 |
+----------------------------------+-----------+
1 row in set (0.02 sec)
select count(distinct concat('c',content)),count(*) from im_message_201001_11 where owner='huaniaoyuchong83' and time between '2010-01-01 00:00:00' and '2010-01-05 00:00:00' ;
+-------------------------------------+----------+
| count(distinct concat('c',content)) | count(*) |
+-------------------------------------+----------+
| 3679 | 4097 |
+-------------------------------------+----------+
1 row in set (0.74 sec)
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 136916032 |
+----------------------------------+-----------+
1 row in set (0.01 sec)
select 136916032-136742194;
+---------------------+
| 136916032-136742194 |
+---------------------+
| 173838 |
+---------------------+
1 row in set (0.00 sec)
#######开始第三次测试
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 136916033 |
+----------------------------------+-----------+
1 row in set (0.01 sec)
select count(distinct concat('c',content)),count(*) from im_message_201001_11 where owner='huaniaoyuchong83' and time between '2010-01-01 00:00:00' and '2010-01-01 00:00:00' ;
+-------------------------------------+----------+
| count(distinct concat('c',content)) | count(*) |
+-------------------------------------+----------+
| 0 | 0 |
+-------------------------------------+----------+
1 row in set (0.85 sec)
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 137086323 |
+----------------------------------+-----------+
1 row in set (0.01 sec)
select 137086323-136916033;
+---------------------+
| 137086323-136916033 |
+---------------------+
| 170290 |
+---------------------+
1 row in set (0.00 sec)
#######开始第四次测试
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 137086324 |
+----------------------------------+-----------+
1 row in set (0.02 sec)
select count(*) from im_message_201001_11 where owner='huaniaoyuchong83' and time between '2010-01-01 00:00:00' and '2010-02-01 00:00:00' ;
+----------+
| count(*) |
+----------+
| 44397 |
+----------+
1 row in set (0.05 sec)
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 137092204 |
+----------------------------------+-----------+
1 row in set (0.01 sec)
select 137092204-137086324 ;
+---------------------+
| 137092204-137086324 |
+---------------------+
| 5880 |
+---------------------+
1 row in set (0.00 sec)
#######开始第五次测试
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 137092205 |
+----------------------------------+-----------+
1 row in set (0.01 sec)
select count(*) from im_message_201001_11 where owner='huaniaoyuchong83' ;
+----------+
| count(*) |
+----------+
| 44397 |
+----------+
1 row in set (0.04 sec)
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 137098085 |
+----------------------------------+-----------+
1 row in set (0.01 sec)
select 137098085-137092205 ;
+---------------------+
| 137098085-137092205 |
+---------------------+
| 5880 |
+---------------------+
1 row in set (0.00 sec)
#######开始第六次测试
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 137098131 |
+----------------------------------+-----------+
1 row in set (0.02 sec)
select count(*) from im_message_201001_11 where owner='huaniaoyuchong83' and time between '2010-01-01 00:00:00' and '2010-01-05 00:00:00' ;
+----------+
| count(*) |
+----------+
| 4097 |
+----------+
1 row in set (0.05 sec)
show session status like 'Innodb_buffer_pool_read_requests';
+----------------------------------+-----------+
| Variable_name | Value |
+----------------------------------+-----------+
| Innodb_buffer_pool_read_requests | 137104011 |
+----------------------------------+-----------+
1 row in set (0.01 sec)
select 137104011-137098131;
+---------------------+
| 137104011-137098131 |
+---------------------+
| 5880 |
+---------------------+
1 row in set (0.00 sec)
####### 分析结果
前三次查询，从索引检索后需要回表：
time 结果行数 逻辑读
30天 44397 176117
5天 4097 173838
1天 0 170290
后三次查询，从索引检索后不需要回表
time 结果行数 逻辑读
30天 44397 5880
无time条件 44397 5880
5天 4097 5880
从数据来看，
select count(*) 这样的查询，有或者没有time条件，逻辑读是一样，都不用回表。
这里也说明这种情况MYSQL是用索引进行time字段的过滤。
select count(distinct concat('c',content)),count(*), 这样的查询，用到了索引以外的字段，是必需回表的。
但通过逻辑读发现，不管查询结果是多少行，逻辑读都差不多，在17W左右。
特别是结果行为0时，如果是通过索引过滤time，那么逻辑读应该接近5900,而不是17W。
这也说明，这种情况下，MYSQL没有使用索引来对TIME字段进行过滤;
所以MYSQL对相同WHERE条件的查询，还采用了不同的优化程序；但MS这个优化有点问题。
对这样的索引，需要优化，可以。 调整索引顺序(`owner`,`time`,`other`)。
但是这仅仅是对一个SQL的优化。
你还要考虑到系统里还有很多其他类似的SQL需要用到这个索引。 所以在优化时，需要评估所有的SQL。
