Database management system

A database management system is software that defines, stores, queries and updates databases while managing access, integrity and concurrent use. Its components can include a query parser, optimizer, storage manager, transaction manager and recovery mechanism. A database is the managed collection; the database management system supplies the mechanisms and interfaces used to operate on it.

For example, evaluating SELECT name FROM Person WHERE id = 7 requires interpreting the query, choosing an access path, retrieving visible records and returning the selected field. An index can speed the lookup without changing its logical result. Concurrent updates may require locks or versions, and a crash may require log replay. Merely writing records to a file does not supply those guarantees. PostgreSQL's tutorial illustrates the distinction between client applications, the server process managing database files and connections, and the databases accessed through that server.^[ref]