Difference between revisions of "SQL Indexing"
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Revision as of 06:56, 2 October 2014
These are notes that I took about SQL Indexing from http://use-the-index-luke.com
Introduction
- An index makes a SQL query fast.
- An index is a distinct structure in the DB that requires its own space.
- A DB index is similar to index of a book - key concept is all entries are arranged in a well-defined order. Finding data in an ordered set is fast and easy because the sort order determines each entries position.
- A DB index however undergoes constant change. Whenever INSERT, UPDATE, DELETE's are executed, the index must also be updated without moving around large amounts of data.
- The DB combines two data structures for providing indexing - doubly linked lists and search trees.
- Doubly linked list enables DB to read indexes forwards and backwards. Index leaf nodes store the indexes in a DB block or page. The blocks are logically stored in the doubly linked list.
- The Index Leaf nodes however are stored in an arbitrary order on the disk - not according to logical order. Therefore the DB needs a second data structure to find the index leaf nodes quickly.
- This is the Balanced Tree : B-Tree. The Branch nodes store
http://use-the-index-luke.com/img/fig01_02_tree_structure.en.svg
