Oracle Database 中 B*Tree 索引内部维护

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 |
| |
|-----------------------------------------------------| <- PCTFREE say 10
| |
| Index Key Data |
| |
| |
| |
| |
| |
| |
| |
| |
| |
|-----------------------------------------------------|


B*tree Index Creation