What is an OID? What is a CTID?
If a table is created WITH OIDS, each row gets a unique a OID. OIDs are automatically assigned unique 4-byte integers that are unique across the entire installation. However, they overflow at 4 billion, and then the OIDs start being duplicated. PostgreSQL uses OIDs to link its internal system tables together. To uniquely number rows in user tables, it is best to use SERIAL rather than OIDs because SERIAL sequences are unique only within a single table. and are therefore less likely to overflow. SERIAL8 is available for storing eight-byte sequence values. CTIDs are used to identify specific physical rows with block and offset values. CTIDs change after rows are modified or reloaded. They are used by index entries to point to physical rows.
