A column is either a user-defined column of a given table or one of the following system-defined columns: oid, tableoid, xmin, cmin, xmax, cmax, ctid. Of these, only oid and tableoid are of interest to database users.
The object identifier (object ID) of a row. This is a serial number that is automatically added by PostgreSQL to all table rows (unless the table was created WITHOUT OIDS, in which case this column is not present).
The OID of the table containing this row. This attribute is particularly useful for queries that select from inheritance hierarchies, since without it, it is difficult to tell which individual table a row came from. The tableoid can be joined against the oid column of pg_class to obtain the table name.