Index (database)

A database index is any data structure that improves the performance of lookup and sorting compared to a full table scan for example.

There are many different data structures used for this purpose, partly depending on the type of database. In a traditional SQL relational DB, for example, this could be thought of as making a column - the index column - faster to search than an unindexed version of the column, thereby making access to the whole row faster.The benefits of indexing is not without cost. There are trade-offs involving lookup performance, index size, and index-update performance. As so often with design, this means that the best way to set up your indexing depends on your intended usage and resources.

Different types of database (eg traditional SQL DBs or Relational DB Management Systems (RDBMS), and noSQL DBs such as Document DBs) also have different limitations regarding indices. Some Document DBs, for example, may not return any data to a query if that data is not indexed.