Index Full Scans和Fast Full Index Scans的区别
Index Full Scans
A full scan is available if a predicate references one of the columns in the index. The predicate does not need to be an index driver. A full scan is also available when there is no predicate, if both the following conditions are met:All of the columns in the table referenced in the query are included in the index.At least one of the index columns is not null.
A full scan can be used to eliminate a sort operation, because the data is ordered by the index key. It reads the blocks singly.
Fast Full Index Scans
Fast full index scans are an alternative to a full table scan when the index contains all the columns that are needed for the query, and at least one column in the index key has the NOT NULL constraint. A fast full scan accesses the data in the index itself, without accessing the table. It cannot be used to eliminate a sort operation, because the data is not ordered by the index key. It reads the entire index using multiblock reads, unlike a full index scan, and can be parallelized.
Fast full scan is available only with the CBO. You can specify it with the initialization parameter OPTIMIZER_FEATURES_ENABLE or the INDEX_FFS hint. Fast full index scans cannot be performed against bitmap indexes.
A fast full scan is faster than a normal full index scan in that it can use multiblock I/O and can be parallelized just like a table scan.
建议继续学习:
- 由浅入深探究mysql索引结构原理、性能分析与优化 (阅读:15216)
- 浅谈MySQL索引背后的数据结构及算法 (阅读:10008)
- 由浅入深理解索引的实现(2) (阅读:6532)
- HBase二级索引与Join (阅读:5885)
- 如何建立合适的索引? (阅读:5512)
- InnODB和MyISAM索引统计集合 (阅读:5357)
- Innodb 表和索引结构 (阅读:4896)
- mysql查询中利用索引的机制 (阅读:4901)
- MySQL索引背后的数据结构及算法原理 (阅读:4519)
- mysql索引浅析 (阅读:4172)
扫一扫订阅我的微信号:IT技术博客大学习
- 作者:Incessant 来源: Incessant
- 标签: 索引
- 发布时间:2009-10-11 22:24:35
- [55] WEB系统需要关注的一些点
- [50] Oracle MTS模式下 进程地址与会话信
- [48] Go Reflect 性能
- [47] find命令的一点注意事项
- [47] 如何拿下简短的域名
- [46] 图书馆的世界纪录
- [46] Twitter/微博客的学习摘要
- [45] android 开发入门
- [45] IOS安全–浅谈关于IOS加固的几种方法
- [43] 流程管理与用户研究