9.17. 序列函式
This section describes functions for operating on_sequence 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 |
|---|---|---|
currval(regclass) | bigint | Return value most recently obtained with nextvalfor specified sequence |
lastval() | bigint | Return value most recently obtained with nextvalfor 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_calledflag |
The sequence to be operated on by a sequence function is specified by a
regclassargument, which is simply the OID of the sequence in thepg_classsystem catalog. You do not have to look up the OID by hand, however, since theregclassdata 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 ordinarySQLnames, 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
BeforePostgreSQL8.1, the arguments of the sequence functions were of type
text, 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 fromtexttoregclassbefore 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 atextconstant instead ofregclass:nextval('foo'::text)
foo
is looked up at runtime
Note that late binding was the only behavior supported inPostgreSQLreleases 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 execute
nextvalconcurrently, each will safely receive a distinct sequence value.If a sequence object has been created with default parameters, successive
nextvalcalls will return successive values beginning with 1. Other behaviors can be obtained by using special parameters in theCREATE SEQUENCEcommand; see its command reference page for more information.To avoid blocking 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 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 anINSERTwith anON CONFLICTclause will compute the to-be-inserted tuple, including doing any requirednextvalcalls, before detecting any conflict that would cause it to follow theON CONFLICTrule instead. Such cases will leave unused“holes”in the sequence of assigned values. Thus,PostgreSQLsequence objects_cannot be used to obtain“gapless”sequences_.This function requires
USAGEorUPDATEprivilege on the sequence.currvalReturn the value most recently obtained by
nextvalfor this sequence in the current session. (An error is reported ifnextvalhas 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 executednextvalsince the current session did.This function requires
USAGEorSELECTprivilege on the sequence.lastvalReturn the value most recently returned by
nextvalin the current session. This function is identical tocurrval, except that instead of taking the sequence name as an argument it refers to whichever sequencenextvalwas most recently applied to in the current session. It is an error to calllastvalifnextvalhas not yet been called in the current session.This function requires
USAGEorSELECTprivilege on the last used sequence.setvalReset the sequence object's counter value. The two-parameter form sets the sequence's
last_valuefield to the specified value and sets itsis_calledfield totrue, meaning that the nextnextvalwill advance the sequence before returning a value. The value reported bycurrvalis also set to the specified value. In the three-parameter form,is_calledcan be set to eithertrueorfalse.truehas the same effect as the two-parameter form. If it is set tofalse, the nextnextvalwill return exactly the specified value, and sequence advancement commences with the followingnextval. Furthermore, the value reported bycurrvalis 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
setvalis just the value of its second argument.Because sequences are non-transactional, changes made by
setvalare not undone if the transaction rolls back.This function requires
UPDATEprivilege on the sequence.