Oracle Database 中 B*Tree 索引内部维护
当一个btree 索引使用create index语句创建的时候,可以设置pctfree 参数。pctfree指定index block中为了未来更新或者新增索引数
最近一周在复习索引相关的东西,,除了回顾concept,还在MOS上看到了一篇比较好的文档。分享给大家。
文档编号:[ID 30405.1]
This article is only concerned with B*tree indexes which are currently the most commonly used. The theory of B*tree indexes is beyond the scope of this article; for more information refer to computer science texts dealing with data structures.
这篇文档只描述关于当前最常用的b*tree索引。b*tree索引的原理已经超过了本文档的范围。更多的信息可以去查看计算机的数据结构原理。
Format of Index Blocks
~~~~~~~~~~~~~~~~~~~~~~
索引的结构(格式?)
Within a B*tree index, index blocks are either branch blocks, the upper blocks within the B*tree index, or leaf blocks, the lowest level index blocks. Branch blocks contain index data that point to lower level index blocks. Leaf blocks contain every indexed data value and a corresponding ROWID used to locate the actual row.
在b*tree索引中,一共有两种索引块,一种是branch block(分支块),还有leaf block(叶块),一种是高level,一种低level的(低level,在索引底部)。branch block包含定位低等级的索引块(可能是branch block或者leaf block)的pointer 。leaf block包含每一个索引数据值和相应的rowid(用来定位真正的row)。
以下是一个索引块的分布情况:
Index Block Format
|-----------------------------------------------------|
| |
| Index Block Header |
| |
------------------------------------------------------|
| |
| Space reserved for future updates |
| and inserts of new rows with the |
| appropriate key values |
| |
|-----------------------------------------------------| | |
| Index Key Data |
| |
| |
| |
| |
| |
| |
| |
| |
| |
|-----------------------------------------------------|
B*tree Index Creation
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