Difference between revisions of "SQL Indexing"
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* 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. | * 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. | * 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. | + | * 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. |
| + | [[File:index-leaf-nodes.jpg]] | ||
Revision as of 04:30, 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.
