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
, each
named reg
.
Table 8.26 shows an
overview.
something
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 |
regclass | pg_class | relation name | pg_type |
regcollation | pg_collation | collation name | "POSIX" |
regconfig | pg_ts_config | text search configuration | english |
regdictionary | pg_ts_dict | text search dictionary | simple |
regnamespace | pg_namespace | namespace name | pg_catalog |
regoper | pg_operator | operator name | + |
regoperator | pg_operator | operator with argument types | *(integer,integer)
or -(NONE,integer) |
regproc | pg_proc | function name | sum |
regprocedure | pg_proc | function with argument types | sum(int4) |
regrole | pg_authid | role name | smithee |
regtype | pg_type | data type name | integer |
All of the OID alias types for objects that are 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.
For example, myschema.mytable
is acceptable input
for regclass
(if there is such a table). That value
might be output as myschema.mytable
, or
just mytable
, depending on the current search path.
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.
The input functions for these types allow whitespace between tokens,
and will fold upper-case letters to lower case, except within double
quotes; this is done to make the syntax rules similar to the way
object names are written in SQL. Conversely, the output functions
will use double quotes if needed to make the output be a valid SQL
identifier. For example, the OID of a function
named Foo
(with upper case F
)
taking two integer arguments could be entered as
' "Foo" ( int, integer ) '::regprocedure
. The
output would look like "Foo"(integer,integer)
.
Both the function name and the argument type names could be
schema-qualified, too.
Many built-in PostgreSQL functions accept
the OID of a table, or another kind of database object, and for
convenience are declared as taking regclass
(or the
appropriate OID alias type). This means you do not have to look up
the object's OID by hand, but can just enter its name as a string
literal. For example, the nextval(regclass)
function
takes a sequence relation's OID, so you could call it like this:
nextval('foo') operates on sequencefoo
nextval('FOO') same as above nextval('"Foo"') operates on sequenceFoo
nextval('myschema.foo') operates onmyschema.foo
nextval('"myschema".foo') same as above nextval('foo') searches search path forfoo
When you write the argument of such a function as an unadorned
literal string, it becomes a constant of type regclass
(or the appropriate type).
Since this is really just an OID, it will track the originally
identified object despite later renaming, schema reassignment,
etc. This “early binding” behavior is usually desirable for
object references in column defaults and views. But sometimes you might
want “late binding” where the object reference is resolved
at run time. To get late-binding behavior, force the constant to be
stored as a text
constant instead of regclass
:
nextval('foo'::text) foo
is looked up at runtime
The to_regclass()
function and its siblings
can also be used to perform run-time lookups. See
Table 9.74.
Another practical example of use of regclass
is to look up the OID of a table listed in
the information_schema
views, which don't supply
such OIDs directly. One might for example wish to call
the pg_relation_size()
function, which requires
the table OID. Taking the above rules into account, the correct way
to do that is
SELECT table_schema, table_name, pg_relation_size((quote_ident(table_schema) || '.' || quote_ident(table_name))::regclass) FROM information_schema.tables WHERE ...
The quote_ident()
function will take care of
double-quoting the identifiers where needed. The seemingly easier
SELECT pg_relation_size(table_name) FROM information_schema.tables WHERE ...
is not recommended, because it will fail for tables that are outside your search path or have names that require quoting.
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
, so the system will not let the sequence
be dropped without first removing the default expression. The
alternative of nextval('my_seq'::text)
does not
create a dependency.
(regrole
is an exception to this property. Constants of this
type are not allowed in stored 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.
In some contexts, a 64-bit variant xid8
is used. Unlike
xid
values, xid8
values increase strictly
monotonically and cannot be reused in the lifetime of a database
cluster. See Section 68.1 for more details.
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 5.6.)