9.16. Sequence Manipulation Functions
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 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 sequencefoo
nextval('FOO') operates on sequencefoo
nextval('"Foo"') operates on sequenceFoo
The sequence name can be schema-qualified if necessary:
nextval('myschema.foo') operates onmyschema.foo
nextval('"myschema".foo') same as above nextval('foo') searches search path forfoo
See Section 8.18 for more information about regclass
.
Note
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
orUPDATE
privilege on the sequence.currval
Return the value most recently obtained by
nextval
for this sequence in the current session. (An error is reported ifnextval
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 executednextval
since the current session did.This function requires
USAGE
orSELECT
privilege on the sequence.lastval
Return the value most recently returned by
nextval
in the current session. This function is identical tocurrval
, except that instead of taking the sequence name as an argument it refers to whichever sequencenextval
was most recently applied to in the current session. It is an error to calllastval
ifnextval
has not yet been called in the current session.This function requires
USAGE
orSELECT
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 itsis_called
field totrue
, meaning that the nextnextval
will advance the sequence before returning a value. The value reported bycurrval
is also set to the specified value. In the three-parameter form,is_called
can be set to eithertrue
orfalse
.true
has the same effect as the two-parameter form. If it is set tofalse
, the nextnextval
will return exactly the specified value, and sequence advancement commences with the followingnextval
. Furthermore, the value reported bycurrval
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); Nextnextval
will return 42The result returned by
setval
is just the value of its second argument.This function requires
UPDATE
privilege on the sequence.
Caution
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.