# Index - Btree structure (Balanced Tree)

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### Table of Contents

## 1 - About

B*Tree indexes :

- has a tree structure
- provides fast access, by key, to an individual row or range of rows
- normally requiring few reads to find the correct row
- are similar in implementation to a binary search tree
- there is a one-to-one relationship between an index entry and a row: an entry points to a row. Not the case of Relation - Bitmap Indexes.

- B in B*tree does not stand for binary but rather for balanced.
- A B*tree is not a binary tree at all

## 2 - Articles Related

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## 3 - Structure

The lowest level blocks in the tree, called leaf nodes or leaf blocks, contain every indexed key and a rowid that points to the row it is indexing. The interior blocks, above the leaf nodes are known as **branch blocks**. They are used to navigate through the structure.

That makes satisfying a predicate, such as the followings, pretty simple :

- where x between 20 and 30
- where x = 30

All leaf blocks should be at the same level.