What is a Database Cluster?
A database cluster is a collection of databases looked after by one PostgreSQL server. Don't get confused - a database cluster isn't a group of database servers. Just one server runs on one computer and deals with one database cluster.
Stepping into the Database Cluster
Each database within a cluster in PostgreSQL is logically separated from the others. This implies that each database has a unique set of database objects that only belong to that database. A specific database's tables, indexes, and other objects, for instance, are only accessible within that database.
Object Identifiers (OIDs)
Each database object in PostgreSQL is managed by an object identifier (OID), a 4-byte unsigned integer. Depending on the type of object, the relationships between database objects and the corresponding OIDs are stored in the appropriate system catalogues. For instance, heap tables' _and _databases' OIDs are stored in the respective pg_database and pg_class.
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