[基础]索引
索引的评价因素：
1） 访问类型，如
records with a specified value in the attribute
or records with an attribute value falling in a specified range of values.
2）访问时间
3）插入时间
4）删除时间
5）空间开销
索引的类型：
1）有序索引
1.1.1）索引顺序组织
随着文件的增大，索引查找性能和顺序扫描性能都会下降，但是这种性能的下降可以通过对文件进行重新组织来弥补。
1.1.2）B+树组织
-Advantage：automatically reorganizes itself with small, local, changes, in the face of insertions and deletions. Reorganization of entire file is not required to maintain performance.
-Disadvantage: extra insertion and deletion overhead, space overhead.
2）散列索引
Comparison of Ordered Indexing and Hashing
1） Cost of periodic re-organization
2）Relative frequency of insertions and deletions
3）Is it desirable to optimize average access time at the expense of worst-case access time?
4）Expected type of queries:
- Hashing is generally better at retrieving records having a specified value of the key.
-If range queries are common, ordered indices are to be preferred
通常设计者会使用有序索引，除非他预先知道将来不会频繁是有范围查询。
多码访问
1）使用多个单码访问
2）多码索引
3）位图索引
1） 访问类型，如
records with a specified value in the attribute
or records with an attribute value falling in a specified range of values.
2）访问时间
3）插入时间
4）删除时间
5）空间开销
索引的类型：
1）有序索引
1.1.1）索引顺序组织
随着文件的增大，索引查找性能和顺序扫描性能都会下降，但是这种性能的下降可以通过对文件进行重新组织来弥补。
1.1.2）B+树组织
-Advantage：automatically reorganizes itself with small, local, changes, in the face of insertions and deletions. Reorganization of entire file is not required to maintain performance.
-Disadvantage: extra insertion and deletion overhead, space overhead.
2）散列索引
Comparison of Ordered Indexing and Hashing
1） Cost of periodic re-organization
2）Relative frequency of insertions and deletions
3）Is it desirable to optimize average access time at the expense of worst-case access time?
4）Expected type of queries:
- Hashing is generally better at retrieving records having a specified value of the key.
-If range queries are common, ordered indices are to be preferred
通常设计者会使用有序索引，除非他预先知道将来不会频繁是有范围查询。
多码访问
1）使用多个单码访问
2）多码索引
3）位图索引
来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/58054/viewspace-622086/ ，如需转载，请注明出处，否则将追究法律责任。
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