8.9. Sequence Manipulation Functions

This section describes EnterpriseDB'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 8-28, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.

There are two ways to operate on sequences in EnterpriseDB. The historical manner described below and the redwood compatible manner discussed later in the chapter Section 8.9.1.

Table 8-28. 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 lowercase 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:

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.) Notice that because this is returning a session-local value, it gives a predictable answer even if other sessions are executing nextval meanwhile.

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. 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) FROM DUAL;           Next nextval will return 43
SELECT setval('foo', 42, true) FROM DUAL;     Same as above
SELECT setval('foo', 42, false) FROM DUAL;    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 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 may leave unused "holes" in the sequence of assigned values. setval operations 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.

8.9.1. Redwood Compatible

EnterpriseDB also provides redwood compatible sequence operations. A sequence can be operated upon in the following manner as well:

sequence.CURRVAL

sequence.NEXTVAL

Hence we have the option to call the next value of the sequence 'foo' in the following manner:

   SELECT foo.nextval;

The above statement will have the same effect as the statement:

   SELECT nextval('foo');

The following example will demonstrate the prowess of sequences:

  CREATE SEQUENCE foo;

  CREATE TABLE test_tb ( a integer, b integer default foo.nextval);

  INSERT INTO test_tb (a) values (1);
  INSERT INTO test_tb (a) values (1);
  INSERT INTO test_tb (a) values (1);

Here we have created a sequence foo, and a table test_tb. We have made three insertions in the table. We will get the following result, once we execute the SQL statement.

  SELECT * FROM test_tb;

The result will be:

   a    |     b
--------|-----------   
   1    |     1
   1    |     2
   1    |     3