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PostgreSQL is a relational
database management system (RDBMS). That means it is a system
for managing data stored in relations. Relation is essentially a
mathematical term for table. The notion of storing data in tables is
so commonplace today that it might seem inherently obvious, but there are a number of other
ways of organizing databases. Files and directories on Unix-like operating systems form an
example of a hierarchical database. A more modern development is the object-oriented database.
Each table is a named collection of rows. Each row of a given table has the same set of named columns, and each column is of a specific data type. Whereas columns
have a fixed order in each row, it is important to remember that SQL does not guarantee the
order of the rows within the table in any way (although they can be explicitly sorted for
display).
Tables are grouped into databases, and a collection of databases
managed by a single PostgreSQL server instance constitutes a
database cluster.
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