Index.dat

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In the Microsoft Windows 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.

Note: .dat is a commonly used filename extension for a number of different types of data files (a file that is not human-readable or that does not hold a document-based binary file). Thus one might well encounter a file named index.dat that is not a part of the Windows operating system or of Internet Explorer.

[edit] Controversy

Internet privacy groups contend that the use of index.dat files in the Windows operating system is an invasion of privacy. One of their main complaints is that the index.dat files cannot be deleted or erased easily, because they are always open when Windows (usually the explorer.exe process) is running. Open or "locked" files cannot be deleted in any way when the process using them is running. Also, when this file grows larger than 80MB in size degraded web performance will occur.

Another contention is that the operating system gives a false sense of security. Even after the user has cleared the internet cache folder, temporary internet files folder, and history folder, the index.dat files on Windows continue to store all visited web addresses and cookies and some temporary files. Some people state that this will eventually cause the index.dat files to grow very large, while the average user remains unaware of what is going on.

However, Microsoft representatives have pointed out that the index.dat files can optionally be deleted by advanced users, when Windows is rebooted in Safe Mode. Because Windows does not lock the file in safe mode, the standard "del" command on the index.dat file or the "deltree" command on the directory containing it will delete the file.

In addition, there is software available that will delete these files for less advanced users.

This problem has been fixed in Internet Explorer 7.