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PostgreSQL Object Identifier Data Types

  • Object identifiers (OIDs) are used internally by PostgreSQL as primary keys for various system tables. OIDs are not added to user-created tables, unless WITH OIDS is specified when the table is created, or the default_with_oids configuration variable is enabled. Type oid represents an object identifier. There are also several alias types for oidregprocregprocedure,regoperregoperatorregclassregtyperegroleregnamespaceregconfig, and regdictionary
  • 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, to examine the pg_attribute rows related to a table mytable, one could write:
SELECT * FROM pg_attribute WHERE attrelid = 'mytable'::regclass; 
rather than:
SELECT * FROM pg_attribute
  WHERE attrelid = (SELECT oid FROM pg_class WHERE relname = 'mytable');
While that doesn't look all that bad by itself, it's still oversimplified. A far more complicated sub-select would be needed to select the right OID if there are multiple tables named mytable in different schemas. The regclass input converter handles the table lookup according to the schema path setting, and so it does the "right thing" automatically. Similarly, casting a table's OID to regclass is handy for symbolic display of a numeric OID.
 Object Identifier Types:
Name
References
Description
Value Example
oid
any
numeric object identifier
564182
regproc
pg_proc
function name
sum
regprocedure
pg_proc
function with argument types
sum(int4)
regoper
pg_operator
operator name
+
regoperator
pg_operator
operator with argument types
*(integer,integer) or -(NONE,integer)
regclass
pg_class
relation name
pg_type
regtype
pg_type
data type name
integer
regrole
pg_authid
role name
smithee
regnamespace
pg_namespace
namespace name
pg_catalog
regconfig
pg_ts_config
text search configuration
english
regdictionary
pg_ts_dict
text search dictionary
simple
  • All of the OID alias types for objects grouped by namespace 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 are more appropriate. For regoperator, unary operators are identified by writing NONE for the unused operand.
  • An additional property of most of the OID alias types is the creation of dependencies. If a constant of one of these types appears in a stored expression (such as a column default expression or view), it creates a dependency on the referenced object. For example, if a column has a default expression nextval('my_seq'::regclass)PostgreSQL understands that the default expression depends on the sequence my_seq; the system will not let the sequence be dropped without first removing the default expression. regrole is the only exception for the property. Constants of this type are not allowed in such expressions.
  • 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.

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