复合索引和单列索引的区别？

ShineCQY

最近一直在埋头做事情，静下心来，猛然回首，却发现关于B* tree 索引很多地方不理解。复合索引和单列索引的区别？ 我的理解比较肤浅：复合索引比单列索引效率低，而且耗用的资源比较多。
求解！求权威资料！

fufuh2o

不一定,在某些情况下,你可以认为复合index可以有效提升表选择率

kevinlin.ora

看一下performance tuning guide 中的Understanding Index Performance部分，应该能稍微了解一下索引：
A composite index contains multiple key columns. Composite indexes can provide additional advantages over single-column indexes:
•Improved selectivity
Sometimes you can combine two or more columns or expressions, each with poor selectivity, to form a composite index with higher selectivity.
•Reduced I/O
If all columns selected by a query are in a composite index, then Oracle Database can return these values from the index without accessing the table.