Editing Relational database (section)

===Index===
{{Main|Index (database)}}
An index is one way of providing quicker access to data. Indices can be created on any combination of attributes on a [[relation (database)|relation]]. Queries that filter using those attributes can find matching tuples directly using the index (similar to [[Hash table]] lookup), without having to check each tuple in turn. This is analogous to using the [[Index (publishing)|index of a book]] to go directly to the page on which the information you are looking for is found, so that you do not have to read the entire book to find what you are looking for. Relational databases typically supply multiple indexing techniques, each of which is optimal for some combination of data distribution, relation size, and typical access pattern. Indices are usually implemented via [[B+ tree]]s, [[R-tree]]s, and [[Bitmap index|bitmaps]].
Indices are usually not considered part of the database, as they are considered an implementation detail, though indices are usually maintained by the same group that maintains the other parts of the database. The use of efficient indexes on both primary and foreign keys can dramatically improve query performance. This is because B-tree indexes result in query times proportional to log(n) where n is the number of rows in a table and hash indexes result in constant time queries (no size dependency as long as the relevant part of the index fits into memory).