What is an OID?
If a table is created WITH OIDS, each row includes an OID column that is automatically filled in during INSERT. OIDs are sequentially assigned 4-byte integers. Initially they are unique across the entire installation. However, the OID counter wraps around at 4 billion, and after that OIDs may be duplicated. It is possible to prevent duplication of OIDs within a single table by creating a unique index on the OID column (but note that the WITH OIDS clause doesn’t by itself create such an index). The system checks the index to see if a newly generated OID is already present, and if so generates a new OID and repeats. This works well so long as no OID-containing table has more than a small fraction of 4 billion rows. PostgreSQL uses OIDs for object identifiers in the system catalogs, where the size limit is unlikely to be a problem. To uniquely number rows in user tables, it is best to use SERIAL rather than an OID column, or BIGSERIAL if the table is expected to have more than 2 billion e
