Reading the internals of Postgres: Database cluster, databases, and tables

PostgreSQL organizes data hierarchically through a database cluster (one instance managing multiple databases), where each database contains objects like tables and indexes identified by unique object identifiers (OIDs). System catalogs such as pg_database and pg_class store metadata about these objects as regular tables, allowing PostgreSQL to manage both built-in and user-defined objects through the same mechanism. All cluster data is physically stored in a single directory specified by the PGDATA environment variable, which is initialized during PostgreSQL installation.