Object identifiers (OIDs) are used internally by
PostgreSQL as primary keys for various
system tables.
Type oid represents an object identifier. There are also
several alias types for oid: regproc,
regprocedure, regoper, regoperator,
regclass, regtype, regrole,
regnamespace, regconfig, and
regdictionary. Table 8.26 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.
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.
Table 8.26. 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.
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
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 5.5.)