= PostgreSQL
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<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.<What Is PostgreSQL? in the PostgreSQL documentation>[Preface §1, origins and supported features]{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.<Concepts in the PostgreSQL documentation>[§2.2, relations, columns and unspecified row order]{ref} These distinctions separate logical results from storage organization and display.
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