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 9.50, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Table 9.50. Sequence Functions
| Function Description | 
|---|
| 
        Advances the sequence object to its next value and returns that value.
        This is done atomically: even if multiple sessions
        execute  
        This function requires  | 
| 
        
         
        Sets the sequence object's current value, and optionally
        its  
SELECT setval('myseq', 42);           Next 
        The result returned by  
        This function requires  | 
| 
        Returns the value most recently obtained
        by  
        This function requires  | 
| 
        Returns the value most recently returned by
         
        This function requires  | 
    To avoid blocking concurrent transactions that obtain numbers from
    the same sequence, the value obtained by nextval
    is not reclaimed for re-use if the calling transaction later aborts.
    This means that transaction aborts or database crashes can result in
    gaps in the sequence of assigned values.  That can happen without a
    transaction abort, too.  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.
    Thus, PostgreSQL sequence
    objects cannot be used to obtain “gapless”
    sequences.
   
    Likewise, sequence state changes made by setval
    are immediately visible to other transactions, and are not undone if
    the calling transaction rolls back.
   
    If the database cluster crashes before committing a transaction
    containing a nextval
    or setval call, the sequence state change might
    not have made its way to persistent storage, so that it is uncertain
    whether the sequence will have its original or updated state after the
    cluster restarts.  This is harmless for usage of the sequence within
    the database, since other effects of uncommitted transactions will not
    be visible either.  However, if you wish to use a sequence value for
    persistent outside-the-database purposes, make sure that the
    nextval call has been committed before doing so.
   
   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.