| PostgreSQL 7.4.30 Documentation | ||||
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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 9-34, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Table 9-34. Sequence Functions
| Function | Return Type | Description |
|---|---|---|
nextval(text) |
bigint | Advance sequence and return new value |
currval(text) |
bigint | Return value most recently obtained with nextval |
setval(text,
bigint) |
bigint | Set sequence's current value |
setval(text,
bigint, boolean) |
bigint | Set sequence's current value and is_called flag |
For largely historical reasons, the sequence to be operated on by a sequence-function call is specified by a text-string argument. To achieve some compatibility with the handling of ordinary SQL names, the sequence functions convert their argument to lower case unless the string is double-quoted. 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
Of course, the text argument can be the result of an expression, not only a simple literal, which is occasionally useful.
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 execute nextval
concurrently, each will safely receive a distinct sequence
value.
currvalReturn 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.) Notice that because this is
returning a session-local value, it gives a predictable
answer even if other sessions are executing nextval meanwhile.
setvalReset 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. In the three-parameter
form, is_called may be set either
true or false. If it's set to false, the next nextval will return exactly the specified
value, and sequence advancement commences with the
following nextval. 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.
Important: To avoid blocking of concurrent transactions that obtain numbers from the same sequence, a
nextvaloperation is never rolled back; that is, once a value has been fetched it is considered used, even if the transaction that did thenextvallater aborts. This means that aborted transactions may leave unused "holes" in the sequence of assigned values.setvaloperations are never rolled back, either.
If a sequence object has been created with default parameters,
nextval calls on it 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.
If you get the error:
ERROR: the_seq.currval is not yet defined in this session
You can use the following sub select to acheive the same result as currval:
SELECT last_value FROM the_seq;
eg:
INSERT INTO a_table VALUES ((SELECT last_value FROM dml_seq));
The query suggested by a previous poster
SELECT last_value FROM the_seq;
might not always do what you want. For example, when the table is brand new, or when setval(x,false) has been called, it will return the next value, not the last value:
CREATE SEQUENCE the_seq MINVALUE 0;
SELECT last_value FROM the_seq; returns 0
SELECT nextval(\'the_seq\'); returns 0
SELECT last_value FROM the_seq; returns 0
The following query is a little smarter:
SELECT CASE WHEN is_called THEN last_value
ELSE last_value-increment_by END from the_seq;
This would return -1 immediately after the table creation above,
and not return 0 until after the first call to nextval(). However, if your sequence ever cycles this probably doesn\'t do the right thing either.
Be warned that if you try updating multiple rows with nextval(), the nextval operator will only be evaluated once at the beginning of the update instead of once for each row. In other words, if you run the following:
UPDATE foo SET bar=nextval('my_sequence');
Then each row's "bar" will be set to the same value, not incrementing values. This is because the "nextval" function is not marked as "volatile", so the parser optimizes it out.
The easiest way to set a field to incrementing values is to do this:
CREATE FUNCTION volatile_nextval(text) RETURNS bigint VOLATILE AS 'BEGIN RETURN nextval($1); END;' LANGUAGE 'plpgsql';
UPDATE foo SET bar=volatile_nextval('my_sequence');
Be warned that the order that rows are updated cannot be controlled, so although this will result in "bar" having unique values for each row, you can't guarantee what order they'll be in.
Using postgresql 7.4.9 (Debian) I see the following behaviour:
my=> select currval('my_seq');
currval
---------
2
(1 row)
my=> update personen_status set my_id = nextval('my_sequence') ;
UPDATE 2
my=> select my_id from my_table;
my_id
-------
3
4
(2 rows)
my=>
Perhaps the table is too small, but I'd have expected the optimizer to always optimize the call away. I tried it with a bigger table (~250000 rows) too and couldn't reproduce JazFresh' problem.