Object identifiers (OIDs) are used internally by PostgreSQL as primary keys for various system tables. Also, an OID system column is added to user-created tables (unless WITHOUT OIDS is specified at table creation time). Type oid represents an object identifier. There are also several alias types for oid: regproc, regprocedure, regoper, regoperator, regclass, and regtype. Table 8-19 shows an overview.
The oid type is currently implemented as an unsigned four-byte integer. Therefore, it is not large enough to provide database-wide uniqueness in large databases, or even in large individual tables. So, using a user-created table's OID column as a primary key is discouraged. OIDs are best used only for references to system tables.
The oid type itself has few operations beyond comparison. It can be cast to integer, however, and then manipulated using the standard integer operators. (Beware of possible signed-versus-unsigned confusion if you do this.)
The OID alias types have no operations of their own except for specialized input and output routines. These routines are able to accept and display symbolic names for system objects, rather than the raw numeric value that type oid would use. The alias types allow simplified lookup of OID values for objects: for example, one may write 'mytable'::regclass to get the OID of table mytable, rather than SELECT oid FROM pg_class WHERE relname = 'mytable'. (In reality, a much more complicated SELECT would be needed to deal with selecting the right OID when there are multiple tables named mytable in different schemas.)
Table 8-19. Object Identifier Types
|oid||any||numeric object identifier||564182|
|regprocedure||pg_proc||function with argument types||sum(int4)|
|regoperator||pg_operator||operator with argument types||*(integer,integer) or -(NONE,integer)|
|regtype||pg_type||data type name||integer|
All of the OID alias types accept schema-qualified names, and will display schema-qualified names on output if the object would not be found in the current search path without being qualified. The regproc and regoper alias types will only accept input names that are unique (not overloaded), so they are of limited use; for most uses regprocedure or regoperator is more appropriate. For regoperator, unary operators are identified by writing NONE for the unused operand.
Another identifier type used by the system is xid, or transaction (abbreviated xact) identifier. This is the data type of the system columns xmin and xmax. Transaction identifiers are 32-bit quantities.
A third identifier type used by the system is cid, or command identifier. This is the data type of the system columns cmin and cmax. Command identifiers are also 32-bit quantities.
A final identifier type used by the system is tid, or tuple identifier (row identifier). This is the data type of the system column ctid. A tuple ID is a pair (block number, tuple index within block) that identifies the physical location of the row within its table.
(The system columns are further explained in Section 5.2.)