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HomeDatabaseMysql TutorialOracle 11g Release (11.1) 索引底层的数据结构

本文介绍关于 Oracle 索引的结构。大概了解 Oracle 索引底层的数据结构,从而更好地理解 Oracle 索引对增、删、改、查的性能

本文内容 B-树(B-tree) 散列(Hash) k-d 树(k-d tree) 点四叉树(Point Quadtree)

本文介绍关于 Oracle 索引的结构。大概了解 Oracle 索引底层的数据结构,从而更好地理解 Oracle 索引对增、删、改、查的性能。

B-树(B-tree)

非索引的结构能满足所有需要,但自平衡的 B-树索引结构更能优化在大数据集上检索的性能。每个 B-树节点拥有多个键和指针。特定 B-树支持的一个节点中键的最大数量是那颗树的顺序。每个节点都具有一个潜在的 order+1 指针,指向比它更低一级的节点。

例如,如图 1 所示,order=2 的 B-树具有三个指针,分别指向:比它第一个键小的子节点(最左边的指针);比它第一个键大,比第二个键小的子节点(中间的指针);比它第二个键大的子节点(最右边的指针)。因此,B-树算法,最大限度地减少定位记录所需的读写,通过传递比二叉树算法更少的节点,二叉树对每个确定的节点,用一个键和最多两个子节点(二叉树的结构是一个键值,左右两个指针,B-树是二叉树的扩展)。下图描述的是克努特变换(Knuth variation),它的索引由两部分组成:一个顺序集(Sequence set),提供快速顺序的访问数据;一个索引集(Index set),提供直接访问顺序集。

虽然,B-树的节点,一般不包含相同数量的数据值,并且他们通常包含一定量的未使用空间,B-树算法确保树保持平衡,和叶节点在同一级上。

图 1 B-树

散列(Hash)

散列根据一个给定字段值快速直接地访问一个特定的已存储的记录。每个记录被放置的位置是根据同一个函数,记录的一些字段域的函数计算的。并用相同的函数插入和更新。

散列的问题是记录的物理顺序与它们的逻辑顺序没有任何关系。另外,散列会在磁盘上存在大量未使用的区域。

图 2 散列

k-d 树(k-d tree)

具有两维的数据,例如经度和纬度,可用通过使用 k-d树变换,称为 2-d 树,被有效地存储和检索。

在这个结构,每个节点的数据类型,是字段信息,两个坐标,和指向两个子节点的左指针和右指针。

图 3 2-d 树

这种结构利于范围查询。也就是说,如果用户指定一个点(xx, xx)和一个距离,那么,查询会返回在这个指定的原来点距离内的所有点集合。

2-d 树很容易实现。但是因为,一个包含 k 个节点的 2-d 树具有 k 高度,因此,插入和查询复杂。

点四叉树(Point Quadtree)

点四叉树,在图 4 所示,也用来表示在一个两维空间中的点数据,但这些结构把区域划分为四个部分,而 2-d 树划分为两个。节点记录类型的字段由属性信息组成,包括两个坐标和指向四个子节点的方位点,按顺时针,如西北NW,西南SW,东北NE,东南SE。

图 4 Point Quadtree 索引结构

点四叉树跟 2-d 树一样也很容易实现。一个包含 k 个节点的四叉树具有 k 高度,插入和查询复杂。每个比较都要求在至少两个坐标上进行。然而,实际中,从 root 到 leaf 的长度在点四叉树中往往较短。

复制上面第二个链接里边提供的 Python 代码,做适当修改。因为,网页提供的代码只能运行在较低版本 Python。Python 3 之后的版本跟之前的差异较大。因此,下载本文最后源代码,并在 Python 3.3 的 IDLE 运行。会得到如下输出:

Python 3.3.0 (v3.3.0:bd8afb90ebf2, Sep 29 2012, 10:57:17) [MSC v.1600 64  (AMD64)]  win32
Type "copyright", "credits"  "license()"  more information.
>>> ================================ RESTART ================================
>>> 
<?xml version="1.0" encoding="iso-8859-1"?>
-
 "http:
<svg xmlns="http:
 <line x1="1" y1="1" x2="1" y2="399"></line>
 <line x1="1" y1="399" x2="399" y2="399"></line>
 <line x1="399" y1="399" x2="399" y2="1"></line>
 <line x1="399" y1="1" x2="1" y2="1"></line>
 <line x1="200" y1="1" x2="200" y2="399"></line>
 <line x1="1" y1="200" x2="399" y2="200"></line>
 <line x1="100" y1="1" x2="100" y2="200"></line>
 <line x1="1" y1="100" x2="200" y2="100"></line>
 <line x1="50" y1="1" x2="50" y2="100"></line>
……

复制输出的结果,命名为 .svg,.html 也行,用浏览器打开,会呈现下图:

图 5 一个 8*8 大小的点四叉树区域

看这个图,从左上角开始,顺时针。你可以当做“根据需要,是否要点,不断按 4 个分裂其中一个方块”。

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