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Relations as Files

PostgreSQL identifies relation files via their OIDs.

The core data structure for this is RelFileNode: 

typedef struct RelFileNode {
    Oid  spcNode;  // tablespace
    Oid  dbNode;   // database
    Oid  relNode;  // relation
} RelFileNode;

Global (shared) tables (e.g. pg_database) have

  •   spcNode == GLOBALTABLESPACE_OID
  •   dbNode == 0