This section describes functions for operating on sequence objects, also called sequence generators or just sequences. Sequence objects are special single-row tables created with CREATE SEQUENCE. Sequence objects are commonly used to generate unique identifiers for rows of a table. The sequence functions, listed in Table 9.47, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Table 9.47. Sequence Functions
| Function | Return Type | Description | 
|---|---|---|
|  | bigint | Return value most recently obtained with nextvalfor specified sequence | 
|  | bigint | Return value most recently obtained with nextvalfor any sequence | 
|  | bigint | Advance sequence and return new value | 
|  | bigint | Set sequence's current value | 
|  | bigint | Set sequence's current value and is_calledflag | 
   The sequence to be operated on by a sequence function is specified by
   a regclass argument, which is simply the OID of the sequence in the
   pg_class system catalog.  You do not have to look up the
   OID by hand, however, since the regclass data type's input
   converter will do the work for you.  Just write the sequence name enclosed
   in single quotes so that it looks like a literal constant.  For
   compatibility with the handling of ordinary
   SQL names, the string will be converted to lower case
   unless it contains double quotes around the sequence name.  Thus:
nextval('foo')      operates on sequence foo
nextval('FOO')      operates on sequence foo
nextval('"Foo"')    operates on sequence Foo
The sequence name can be schema-qualified if necessary:
nextval('myschema.foo')     operates on myschema.foo
nextval('"myschema".foo')   same as above
nextval('foo')              searches search path for foo
   See Section 8.19 for more information about
   regclass.
  
    Before PostgreSQL 8.1, the arguments of the
    sequence functions were of type text, not regclass, and
    the above-described conversion from a text string to an OID value would
    happen at run time during each call.  For backward compatibility, this
    facility still exists, but internally it is now handled as an implicit
    coercion from text to regclass before the function is
    invoked.
   
    When you write the argument of a sequence function as an unadorned
    literal string, it becomes a constant of type regclass.
    Since this is really just an OID, it will track the originally
    identified sequence despite later renaming, schema reassignment,
    etc.  This “early binding” behavior is usually desirable for
    sequence references in column defaults and views.  But sometimes you might
    want “late binding” where the sequence 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
Note that late binding was the only behavior supported in PostgreSQL releases before 8.1, so you might need to do this to preserve the semantics of old applications.
Of course, the argument of a sequence function can be an expression as well as a constant. If it is a text expression then the implicit coercion will result in a run-time lookup.
The available sequence functions are:
nextval
        Advance the sequence object to its next value and return that
        value.  This is done atomically: even if multiple sessions
        execute nextval concurrently, each will safely receive
        a distinct sequence value.
       
        If a sequence object has been created with default parameters,
        successive nextval calls will return successive
        values beginning with 1.  Other behaviors can be obtained by using
        special parameters in the CREATE SEQUENCE command;
        see its command reference page for more information.
       
         To avoid blocking concurrent transactions that obtain numbers from
         the same sequence, a nextval operation is never
         rolled back; that is, once a value has been fetched it is considered
         used and will not be returned again.  This is true even if the
         surrounding transaction later aborts, or if the calling query ends
         up not using the value.  For example an INSERT with
         an ON CONFLICT clause will compute the to-be-inserted
         tuple, including doing any required nextval
         calls, before detecting any conflict that would cause it to follow
         the ON CONFLICT rule instead.  Such cases will leave
         unused “holes” in the sequence of assigned values.
         Thus, PostgreSQL sequence objects cannot
         be used to obtain “gapless” sequences.
        
        This function requires USAGE
        or UPDATE privilege on the sequence.
       
currval
        Return the value most recently obtained by nextval
        for this sequence in the current session.  (An error is
        reported if nextval has never been called for this
        sequence in this session.)  Because this is returning
        a session-local value, it gives a predictable answer whether or not
        other sessions have executed nextval since the
        current session did.
       
        This function requires USAGE
        or SELECT privilege on the sequence.
       
lastval
        Return the value most recently returned by
        nextval in the current session. This function is
        identical to currval, except that instead
        of taking the sequence name as an argument it refers to whichever
        sequence nextval was most recently applied to
        in the current session. It is an error to call
        lastval if nextval
        has not yet been called in the current session.
       
        This function requires USAGE
        or SELECT privilege on the last used sequence.
       
setval
        Reset the sequence object's counter value.  The two-parameter
        form sets the sequence's last_value field to the
        specified value and sets its is_called field to
        true, meaning that the next
        nextval will advance the sequence before
        returning a value.  The value reported by currval is
        also set to the specified value.  In the three-parameter form,
        is_called can be set to either true
        or false.  true has the same effect as
        the two-parameter form. If it is set to false, the
        next nextval will return exactly the specified
        value, and sequence advancement commences with the following
        nextval.  Furthermore, the value reported by
        currval is not changed in this case.  For example,
SELECT setval('foo', 42);           Next nextval will return 43
SELECT setval('foo', 42, true);     Same as above
SELECT setval('foo', 42, false);    Next nextval will return 42
        The result returned by setval is just the value of its
        second argument.
       
         Because sequences are non-transactional, changes made by
         setval are not undone if the transaction rolls
         back.
        
        This function requires UPDATE privilege on the
        sequence.