This section describes functions for operating onsequence objects, also called sequence generators or just sequences. Sequence objects are special single-row tables created withCREATE SEQUENCE. Sequence objects are commonly used to generate unique identifiers for rows of a table. The sequence functions, listed inTable 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 withnextval for specified sequence |
| bigint | Return value most recently obtained withnextval for any sequence |
| bigint | Advance sequence and return new value |
| bigint | Set sequence's current value |
| bigint | Set sequence's current value andis_called flag |
The sequence to be operated on by a sequence function is specified by aregclass argument, which is simply the OID of the sequence in thepg_class system catalog. You do not have to look up the OID by hand, however, since theregclass 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 ordinarySQL names, the string will be converted to lower case unless it contains double quotes around the sequence name. Thus:
nextval('foo')operates on sequencefoonextval('FOO')operates on sequencefoonextval('"Foo"')operates on sequenceFooThe sequence name can be schema-qualified if necessary:
nextval('myschema.foo')operates onmyschema.foonextval('"myschema".foo')same as abovenextval('foo')searches search path forfooSeeSection 8.18 for more information aboutregclass.
BeforePostgreSQL 8.1, the arguments of the sequence functions were of typetext, notregclass, 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 fromtext toregclass before the function is invoked.
When you write the argument of a sequence function as an unadorned literal string, it becomes a constant of typeregclass. 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 atext constant instead ofregclass:
nextval('foo'::text)foo is looked up at runtimeNote that late binding was the only behavior supported inPostgreSQL 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:
nextvalAdvance the sequence object to its next value and return that value. This is done atomically: even if multiple sessions executenextval concurrently, each will safely receive a distinct sequence value.
If a sequence object has been created with default parameters, successivenextval calls will return successive values beginning with 1. Other behaviors can be obtained by using special parameters in theCREATE SEQUENCE command; see its command reference page for more information.
This function requiresUSAGE orUPDATE privilege on the sequence.
currvalReturn the value most recently obtained bynextval 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 requiresUSAGE orSELECT privilege on the sequence.
lastvalReturn the value most recently returned bynextval 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 requiresUSAGE orSELECT privilege on the last used sequence.
setvalReset the sequence object's counter value. The two-parameter form sets the sequence'slast_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);Nextnextval will return 43SELECT setval('foo', 42, true);Same as aboveSELECT setval('foo', 42, false);Nextnextval will return 42The result returned bysetval is just the value of its second argument.
This function requiresUPDATE privilege on the sequence.
To avoid blocking concurrent transactions that obtain numbers from the same sequence, the value obtained bynextval 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 anINSERT with anON CONFLICT clause will compute the to-be-inserted tuple, including doing any requirednextval calls, before detecting any conflict that would cause it to follow theON CONFLICT rule instead. Thus,PostgreSQL sequence objectscannot be used to obtain“gapless” sequences.
Likewise, sequence state changes made bysetval 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 anextval orsetval 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 thenextval call has been committed before doing so.