indexs

An index is a data structure or reference that enables fast data retrieval by providing a mapping from a key value to the location of the corresponding data item. By organizing data to support quick lookups, an index reduces the amount of data that must be scanned during a search.

In books and documents, a traditional index at the end lists topics or terms with page numbers, helping readers locate information without scanning the full text.

In databases, an index is a separate data structure built on one or more columns to speed queries. Common implementations include B-tree and hash indexes. An index can be single-column or composite, and may enforce uniqueness. A cluster index determines the physical order of data rows, while non-cluster indexes reference data stored elsewhere. Proper indexing accelerates search, range queries, and sorted outputs, but incurs storage overhead and maintenance cost when data is updated.

In information retrieval, inverted indexes associate terms with documents and positions, enabling fast full-text search across large collections. Similar indexing concepts are used in search engines, document stores, and file systems.

Designing indexes requires balancing read performance against write overhead and storage use. Indexes are most beneficial on high-selectivity keys and columns used in filtering, joining, or sorting. Over-indexing can degrade performance and complicate maintenance.

The plural of index is commonly indexes or indices, while some contexts tolerate less common variants.