8.19. Object Identifier Types
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.
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.
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 8.26. Object Identifier Types
|any||numeric object identifier|
|text search configuration|
|text search dictionary|
|operator with argument types|
|function with argument types|
|data type name|
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
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.
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 sequence
foonextval('FOO') same as above nextval('"Foo"') operates on sequence
Foonextval('myschema.foo') operates on
myschema.foonextval('"myschema".foo') same as above nextval('foo') searches search path for
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
foois looked up at runtime
to_regclass() function and its siblings can also be used to perform run-time lookups. See Table 9.71.
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 ...
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
xmax. Transaction identifiers are 32-bit quantities. In some contexts, a 64-bit variant
xid8 is used. Unlike
xid8 values increase strictly monotonically and cannot be reused in the lifetime of a database cluster.
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 5.5.)