The fundamental notion in Postgres is that of a table, which is a named collection of rows. Each row has the same set of named columns, and each column is of a specific type. Furthermore, each row has a permanent object identifier (OID) that is unique throughout the database cluster. Historially, tables have been called classes in Postgres, rows are object instances, and columns are attributes. This makes sense if you consider the object-relational aspects of the database system, but in this manual we will use the customary SQL terminology. As previously discussed, tables are grouped into databases, and a collection of databases managed by a single postmaster process constitutes a database cluster.