Flat-file

A flat file database is a database which is stored on its host computer system as an ordinary unstructured file called a "flat file". To access the structure of the data and manipulate it, the file must be read in its entirety into the computer's memory. Upon completion of the database operations, the file is again written out in its entirety to the host's file system. In this stored mode the database is said to be "flat", meaning that it has no structure for indexing and there are usually no structural relationships between the records. A flat file can be a plain text file or a binary file.

The term has generally implied a small, simple database. As computer memory has become cheaper, more sophisticated databases can now be entirely held in memory for faster access. These newer databases would not generally be referred to as flat-file databases.

Plain text files usually contain one record per line, There are different conventions for depicting data. In comma-separated values and delimiter-separated values files, fields can be separated by delimiters such as comma or tab characters. In other cases, each field may have a fixed length; short values may be padded with space characters. Extra formatting may be needed to avoid delimiter collision. More complex solutions are markup languages and programming languages.

Using delimiters incurs some overhead in locating them every time they are processed (unlike fixed-width formatting), which may have performance implications. However, use of character delimiters (especially commas) is also a crude form of data compression which may assist overall performance by reducing data volumes — especially for data transmission purposes. Use of character delimiters which include a length component (Declarative notation) is comparatively rare but vastly reduces the overhead associated with locating the extent of each field.

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