以上是一些普遍的建立索引时的判断依据。一言以蔽之，索引的建立必须慎重，对每个索引的必要性都应该经过仔细分析，要有建立的依
一、B-Tree索引
1. 选择索引字段的原则:
2. 选择建立复合索引
复合索引的优点:
什么情况下优化器会用到复合索引呢?
(a) 当SQL语句的WHERE子句中有用到复合索引的领导字段时,ORACLE优化器会考虑用到复合索引来访问.
(b) 当某几个字段在SQL语句的WHERE子句中经常通过AND操作符联合在一起使用作为过滤谓词,并且这几个字段合在一起时选择性比各自单个字段的选择性要更好时,可
能考虑用这几个字段来建立复合索引.
(c) 当有几个查询语句都是查询同样的几个字段值时,则可以考虑在这几个字段上建立复合索引.
复合索引字段排序的原则:
二、位图索引
什么情况下位图索引能够改善查询的性能呢?
位图索引与B-TREE索引的比较
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