8.18. Object Identifier 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
regdictionary. Table 8.24 shows an overview.
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.
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;
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.
Table 8.24. Object Identifier Types
|any||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
regoper alias types will only accept input names that are unique (not overloaded), so they are of limited use; for most uses
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.
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 (abbreviated xact) 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 in Section 5.4.)