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Oracle index internals explained

Scott Martin, an MIT-trained former Oracle developer has a nice discussion of Oracle index internals:


This article explores the basic architecture of the Oracle B-tree index. An index is an hierarchically organized structure designed to facilitate the rapid lookup of row identifiers (rowids) based on key values. Keys may or may not be unique. Keys may consist of more than one column a "compound" key.  


Indices are structured as a collection of two types of blocks leaf blocks and branch blocks. Leaf blocks contain key/rowid pairs which refer to specific rows in the table. Above the leaf blocks are branch blocks which contain key/dba pairs referring to other index blocks.


One particular block, the root block, is the beginning or root of the index structure. If the index contains few enough keys, the root block may be a leaf block. As the index grows, the root block becomes a branch block.


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