Sequence 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 9-46, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.

Table 9-46. 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 8.18 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.

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 and will not be returned again. This is true even if the surrounding transaction later aborts, or if the calling query ends up not using the value. 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. Such cases will leave unused "holes" in the sequence of assigned values. Thus, PostgreSQL sequence objects cannot be used to obtain "gapless" sequences.

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 refers to whichever sequence nextval was most recently applied to 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. 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: Because sequences are non-transactional, changes made by setval are not undone if the transaction rolls back.

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Links:

https://www.postgresql.org/docs/9.6/static/functions-sequence.html

doc_PostgreSQL

2016-10-12 11:34:08

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