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 | Return Type | Description |
|---|---|---|
| bigint | Return value most recently obtained with
nextval for specified sequence |
| bigint | Return value most recently obtained with
nextval for any sequence |
| bigint | Advance sequence and return new value |
| bigint | Set sequence's current value |
| bigint | Set sequence's current value and is_called flag |
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.
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.
This function requires UPDATE privilege on the
sequence.
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.