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 sequence foo
nextval('FOO')              same as above
nextval('"Foo"')            operates on sequence Foo
nextval('myschema.foo')     operates on myschema.foo
nextval('"myschema".foo')   same as above
nextval('foo')              searches search path for foo
     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 9.71.
    
    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 ...
    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.
   
    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.)