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
The sequence to be operated on by a sequence function is specified by aregclass
argument, which is simply the OID of the sequence in thepg_class
system catalog. You do not have to look up the OID by hand, however, since theregclass
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 ordinarySQLnames, the string will be converted to lower case unless it contains double quotes around the sequence name. Thus:
The sequence name can be schema-qualified if necessary:
SeeSection 8.18for more information aboutregclass
.
Note
BeforePostgreSQL8.1, the arguments of the sequence functions were of typetext
, 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 fromtext
toregclass
before the function is invoked.
When you write the argument of a sequence function as an unadorned literal string, it becomes a constant of typeregclass
. 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 atext
constant instead ofregclass
:
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:
nextval
Advance the sequence object to its next value and return that value. This is done atomically: even if multiple sessions executenextval
concurrently, each will safely receive a distinct sequence value.
If a sequence object has been created with default parameters, successivenextval
calls 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.
Important
To avoid blocking concurrent transactions that obtain numbers from the same sequence, anextval
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 anINSERT
with anON CONFLICT
clause will compute the to-be-inserted tuple, including doing any requirednextval
calls, before detecting any conflict that would cause it to follow theON CONFLICT
rule 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 requiresUSAGE
orUPDATE
privilege on the sequence.
currval
Return the value most recently obtained bynextval
for this sequence in the current session. (An error is reported ifnextval
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 executednextval
since the current session did.
This function requiresUSAGE
orSELECT
privilege on the sequence.
lastval
Return the value most recently returned bynextval
in the current session. This function is identical tocurrval
, except that instead of taking the sequence name as an argument it refers to whichever sequencenextval
was most recently applied to in the current session. It is an error to calllastval
ifnextval
has not yet been called in the current session.
This function requiresUSAGE
orSELECT
privilege on the last used sequence.
setval
Reset the sequence object's counter value. The two-parameter form sets the sequence'slast_value
field to the specified value and sets itsis_called
field totrue
, meaning that the nextnextval
will advance the sequence before returning a value. The value reported bycurrval
is also set to the specified value. In the three-parameter form,is_called
can be set to eithertrue
orfalse
.true
has the same effect as the two-parameter form. If it is set tofalse
, the nextnextval
will return exactly the specified value, and sequence advancement commences with the followingnextval
. Furthermore, the value reported bycurrval
is not changed in this case. For example,
The result returned bysetval
is just the value of its second argument.
Important
Because sequences are non-transactional, changes made bysetval
are not undone if the transaction rolls back.
This function requiresUPDATE
privilege on the sequence.