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 aliases for oid: regproc, regprocedure, regoper, regoperator, regclass, and regtype. Table 5-20 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 (which is implemented as unsigned 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 5-20. Object Identifier Types
|Type name||References||Description||Value example|
|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)|
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.
OIDs are 32-bit quantities and are assigned from a single cluster-wide counter. In a large or long-lived database, it is possible for the counter to wrap around. Hence, it is bad practice to assume that OIDs are unique, unless you take steps to ensure that they are unique. Recommended practice when using OIDs for row identification is to create a unique constraint on the OID column of each table for which the OID will be used. Never assume that OIDs are unique across tables; use the combination of tableoid and row OID if you need a database-wide identifier. (Future releases of PostgreSQL are likely to use a separate OID counter for each table, so that tableoid must be included to arrive at a globally unique identifier.)
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. In a long-lived database it is possible for transaction IDs to wrap around. This is not a fatal problem given appropriate maintenance procedures; see the PostgreSQL 7.3 Administrator's Guide for details. However, it is unwise to depend on uniqueness of transaction IDs over the long term (more than one billion transactions).
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. This creates a hard limit of 232 (4 billion) SQL commands within a single transaction. In practice this limit is not a problem --- note that the limit is on number of SQL commands, not number of tuples processed.
A final identifier type used by the system is tid, or tuple 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 tuple within its table.