Object identifiers (OIDs) are used internally byPostgreSQLas primary keys for various system tables. OIDs are not added to user-created tables, unless
WITH OIDSis specified when the table is created, or thedefault_with_oidsconfiguration variable is enabled. Type
oidrepresents an object identifier. There are also several alias types for
regdictionary.Table 8.24shows an overview.
oidtype 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.
oidtype 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
oidwould use. The alias types allow simplified lookup of OID values for objects. For example, to examine the
pg_attributerows related to a table
mytable, one could write:
SELECT * FROM pg_attribute WHERE attrelid = 'mytable'::regclass;
SELECT * FROM pg_attributeWHERE 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
mytablein different schemas. The
regclassinput 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
regclassis handy for symbolic display of a numeric OID.
Table 8.24. Object Identifier Types
numeric object identifier
function with argument types
operator with argument types
data type name
text search configuration
text search dictionary
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
regoperalias types will only accept input names that are unique (not overloaded), so they are of limited use; for most uses
regoperatorare more appropriate. For
regoperator, unary operators are identified by writing
NONEfor 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),PostgreSQLunderstands 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.
regroleis the only exception for the property. Constants of this type are not allowed in such expressions.
The OID alias types do not completely follow transaction isolation rules. The planner also treats them as simple constants, which may result in sub-optimal planning.
Another identifier type used by the system is
xid, or transaction (abbreviatedxact) identifier. This is the data type of the system columns
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
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 inSection 5.4.)