索引是建立在表上的，本质上是通过索引直接定位表的物理元组，加速数据获取的方式，所以索引优化应该归属到物理查询优化阶段。
1. 如何利用索引
通常查询优化器所使用索引的原则如下：
● 索隐裂座位条件出现在WHERE、HAVING、ON 子句中，这样有利于索引过滤元组；
● 索引列是被链接的表对象的列且存在于连接条件中；
● 还有一些情况可以使用索引，如排序操作、在索引列上球MIN、MAX等。
1.1 示例：
create table A(a1 int unique,a2 varchar(10),a3 int); （a1列上创建隐含索引）
● 对表做查询，没有列对象作为过滤条件（如出现在WHERE子句中），只能顺序扫描
mysql> explain extended select * from A;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | A | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------+
type = ALL 表示 顺序扫描或范围扫描，不是用索引，顺序扫描，直接读取表上的数据（访问数据文件）。
● 对表做查询，有列对象列且索引列作为过滤条件，可作为索引扫描
mysql> explain extended select * from A where a1 >2;
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | A | range | a1 | a1 | 5 | NULL | 1 | 100.00 | Using index condition |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-----------------------+
t ype = range 表示基于索引做范围扫描。Using index condition 表示尝试通过索引来获取
● 对表做查询，索引列被运算符“-” 处理，查询优化器不能再执行前进行取反操作，不可利用索引，只能顺序扫描。
select A.* from A where -A.a1 = -2; (不能利用索引)
select A.* from A where A.a1 = 2; (利用索引)
●对表做查询，选择条件不包括索引列，只能顺序扫描
select A.* from A where A.a3 = 2;
● 索引列参与了运算，不能使用索引
select A.*from A whereA.a1 + A.a3 = 2;
select A.*from A whereA.a1= A.a3 + 2;
select A.*from A whereA.a1< 1+2;（可以使用索引）
● 查询时，索引列对象座位过滤条件时，操作符是 <> 时，不可做索引扫描，如果是 < 或者 > 时，可以使用索引
mysql> explain extended select A.* from A where A.a1 <> 2;(不使用索引)
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | A | ALL | a1 | NULL | NULL | NULL | 2 | 100.00 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
mysql> explain extended select A.* from A where A.a1 > 2;(使用索引)
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | A | range | a1 | a1 | 5 | NULL | 1 | 100.00 | Using index condition |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-----------------------+
● 查询时，索引列对象座位过滤条件时，操作符是 between - and 时，可以使用索引
mysql> explain extended select A.* from A where A.a1 between 1 and 2;
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | A | range | a1 | a1 | 5 | NULL | 2 | 100.00 | Using index condition |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-----------------------+
1.2 所以，对于索引列，索引可用的条件如下：
● 在WHERE, JOIN/ON/HAVING 的条件中出现 “key <op> 常量” 格式的条件子句（索引列不能参与带有变量表达式的运算）
● 操作符不能是<>操作符（不等于操作符在任何类型的列上都不能使用索引）
● 索引列的选择率越低，索引越有效，通常认为选择率小鱼0.1的索引扫描效果会好一些
2. 索引列的位置对使用索引的影响
2.1 对目标列、WHERE 等条件子句的影响
● 索引列出现在目标列通常不可以使用索引（但不是全部情况），索引列出现目标列对查询语句的优化没有好的影响（PostgreSQL），如：
select A.1a from A;（（PostgreSQL））
注意：具体的数据库可能存在差别，在我的mysql环境是可以用到索引的：如下：
mysql> explain extended select A.a1 from A;
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | A | index | NULL | a1 | 5 | NULL | 2 | 100.00 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+----------+-------------+
● 聚集函数MIN/MAX 用在索引列上，出现在目标列中，可以使用索引（PostgreSQL）。如：
select MAX(A.a1) from A;（PostgreSQL）
注意：具体的数据库可能存在差别，在我的mysql环境没有用到索引：如下：
mysql> explain extended select Max(A.a1) from A;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------+()
● 索引出现在JOIN/ON 子句中，作为连接条件，不可使用索引。但是索引列出现限制条件满足“key<op>常量”格式可用到索引。
create table B(b1 int unique,b2 varchar(10),b3 int);
mysql> explain extended select A.*,B.* from A join B on a1 = b1;
+----+-------------+-------+------+---------------+------+---------+-----------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+-----------+------+----------+-------------+
| 1 | SIMPLE | B | ALL | b1 | NULL | NULL | NULL | 1 | 100.00 | Using where |
| 1 | SIMPLE | A | ref | a1 | a1 | 5 | test.B.b1 | 1 | 100.00 | NULL |
+----+-------------+-------+------+---------------+------+---------+-----------+------+----------+-------------+
执行计划表示 对B表进行了全表扫描，对A表连接时使用了索引。
mysql> explain extended select A.*,B.* from A join B on a1 = b1 and a1 = 1;
+----+-------------+-------+-------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+-------+---------------+------+---------+-------+------+----------+-------+
| 1 | SIMPLE | A | const | a1 | a1 | 5 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | B | const | b1 | b1 | 5 | const | 1 | 100.00 | NULL |
+----+-------------+-------+-------+---------------+------+---------+-------+------+----------+-------+
对A和B都使用而索引。
● 还有些关于order by,group by 的，发现数据库不同的话存在差异，就不写了，
当然上面的也可能存在差异，具体的话在用的时候看下执行计划就知道了。
3. 联合索引对索引使用的影响
create table E(e1 int,e2 varchar(10),e3 int,e4 int,primary key(e1,e3,e4));
使用联合索引的前缀部分索引键，可触发索引的使用；
使用部分索引键，但不是联合索引的前缀部分，不可出发索引的使用。
select * from E where e1 = 1;
select * from E where e1 = 1 and e3 =1 ;（select * from E where e1 = 1 or e3 =1 ;这种情况不会用到索引）
select * from E where e1 = 1 and e3 =1 and e4 = 1;
以上三个都是可以使用索引的：
mysql> explain extended select * from E where e1 = 1;
+----+-------------+-------+------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | E | ref | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> explain extended select * from E where e1 = 1 and e3 =1;
+----+-------------+-------+------+---------------+---------+---------+-------------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+---------+---------+-------------+------+----------+-------+
| 1 | SIMPLE | E | ref | PRIMARY | PRIMARY | 8 | const,const | 1 | 100.00 | NULL |
+----+-------------+-------+------+---------------+---------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> explain extended select * from E where e1 = 1 and e3 =1 and e4 = 1;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-----------------------------------------------------+
注意：这里第三个看上去没有使用到索引，那是因为 e1 = 1 and e3 =1这两个条件过滤后就没有数据了，后面的e4=1不用看也知道返回false了。
下面的这种是不会用到索引的：
mysql> explain extended select * from E where e3 = 1 and e4 = 1;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | E | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
另外这种情况：虽然使用到了索引，但是只是使用到了e1这一列上的索引，e4=1 这里没有用到索引。
mysql> explain extended select * from E where e1 = 1 and e4 = 1;
+----+-------------+-------+------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+---------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | E | ref | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using where |
+----+-------------+-------+------+---------------+---------+---------+-------+------+----------+-------------+
4.多个索引对索引使用的影响
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