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A database is an organized collection of data maintained for queries and updates. A database management system manages that collection, enforcing access rules and database integrity constraints. A relational database represents it through database relations, rather than exposing only an arbitrary collection of bytes in files.

For example, a laboratory can store biological sample identifiers, measurement values and scientific instrument identifiers as database tuples. A database query can select measurements above a threshold and join them to scientific instrument records. A foreign key can require every recorded scientific instrument identifier to name an existing scientific instrument. This makes the connection a maintained rule rather than a convention that every application must remember.

The logical relational schema and actual storage are distinct. A database query plan may use an index or scan database pages while returning the same specified result. Database transactions coordinate related updates, and database integrity constraints restrict the permitted states. PostgreSQL's documentation gives a concrete implementation in which named tables contain rows with named, typed columns and databases group tables.^[ref]

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]

PostgreSQL is an open-source object-relational database management system supporting SQL, database transactions, constraints and extensible data types. It manages stored relations and concurrent access through a server, rather than merely defining a database query language. The project descends from the Berkeley POSTGRES system.^[ref]

For example, SELECT name FROM samples WHERE reading > 5 requests selected rows without prescribing their physical access path. A database index can accelerate retrieval, while database transactions govern the visibility and consistency of related updates. A database relation has no implicit presentation order: a caller needing sorted rows must request an ordering.^[ref] These distinctions separate logical results from storage organization and display.

The PostgreSQL Global Development Group is the collective project attribution used for the development and documentation of PostgreSQL, an open-source database management system. Its documentation specifies concrete behavior for database transactions, database integrity constraints, database query operations and storage mechanisms; these specifications are more precise than inferring behavior from SQL syntax alone.^[ref]

For example, PostgreSQL documents transaction isolation levels in terms of visible snapshots and permitted anomalies. The project name here identifies the responsible development community, rather than one individual author or a separate database product.