Sequence Manipulation Functions

PostgreSQL 9.1.2 Documentation
Prev | Up | Chapter 9. Functions and Operators | Next

# 9.15. Sequence Manipulation Functions

This section describes PostgreSQL's functions for operating on sequence objects. Sequence objects (also called sequence generators or just sequences) are special single-row tables created with CREATE SEQUENCE. A sequence object is usually used to generate unique identifiers for rows of a table. The sequence functions, listed in Table 9-39, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.

Table 9-39. Sequence Functions

Function | Return Type | Description
--- | --- | ---
``currval(`regclass`)`` | `bigint` | Return value most recently obtained with `nextval` for specified sequence
``lastval()`` | `bigint` | Return value most recently obtained with `nextval` for any sequence
``nextval(`regclass`)`` | `bigint` | Advance sequence and return new value
``setval(`regclass`, `bigint`)`` | `bigint` | Set sequence's current value
``setval(`regclass`, `bigint`, `boolean`)`` | `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 8.16 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.

`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.

`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 fetches the value of the last sequence used by `nextval` in the current session. It is an error to call `lastval` if `nextval` has not yet been called in the current session.

`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 (this is a change from pre-8.3 behavior). 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.

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.

> Important: 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, even if the transaction that did the `nextval` later aborts. This means that aborted transactions might leave unused "holes" in the sequence of assigned values. `setval` operations are never rolled back, either.

Prev | Home | Next
XML Functions | Up | Conditional Expressions