In the Microsoft Winows operating system, index.dat is a file used by the Internet Explorer web browser. The index.dat file functions as an active database, which runs as long as Windows is active. It functions as a repository of redundant information, such as web URLs, search queries, and recently opened files. Its role is similar to that of an index file in the field of databases, where a technique called "indexing" stores the contents of a database in a different order to help speed up query responses. Similarly, when the Autocomplete function is enabled in Internet Explorer, every web address visited is sorted in the index.dat file, allowing the Internet Explorer to attempt to find an appropriate match when a user types in an edit field. Separate index.dat files exist for the Internet Explorer history, cache, and cookies.