Index logic in MySQL& B- tree algorithm

This blog is about some basic ideas about index logic in the Database engine in MySQL, along with the simple explanation of its supporting algorithm: B- tress algorithm.

B-tree Algorithm

Index logic

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1. B- tree algorithm

http://en.wikipedia.org/wiki/B-tree

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The lower and upper bounds on the number of child nodes are typically fixed for a particular implementation. For example, in a 2-3 B-tree (often simply referred to as a 2-3 tree), each internal node may have only 2 or 3 child nodes.

Each internal node of a B-tree will contain a number of keys. The keys act as separation values which divide its subtrees.

Usually, the number of keys is chosen to vary between d and 2d, where d is the minimum number of keys, and d+1 is the minimum degree or branching factor of the tree.



            Figure1.1 B-tree of order 2

Deleting the key would make the internal node have d-1 keys; joining the neighbor would add d keys plus one more key brought down from the neighbor’s parent. The result is an entirely full node of 2d keys.

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2. Index logic

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        Figure2.1 B-tree index of MySQL5.1

where inner nodes store the pages hierarchy and the leaf nodes store pointers to indexed data information.