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>Sequence Manipulation Functions</TITLE
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><H1
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><A
NAME="FUNCTIONS-SEQUENCE"
>9.12. Sequence Manipulation Functions</A
></H1
><A
NAME="AEN12429"
></A
><A
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></A
><A
NAME="AEN12433"
></A
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></A
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><P
> This section describes <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s functions
for operating on <I
CLASS="FIRSTTERM"
>sequence objects</I
>.
Sequence objects (also called sequence generators or
just sequences) are special single-row tables created with
<TT
CLASS="COMMAND"
>CREATE SEQUENCE</TT
>. A sequence object is usually used to
generate unique identifiers for rows of a table. The sequence functions,
listed in <A
HREF="functions-sequence.html#FUNCTIONS-SEQUENCE-TABLE"
>Table 9-34</A
>,
provide simple, multiuser-safe methods for obtaining successive
sequence values from sequence objects.
</P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-SEQUENCE-TABLE"
></A
><P
><B
>Table 9-34. Sequence Functions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Function</TH
><TH
>Return Type</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>nextval</CODE
>(<TT
CLASS="TYPE"
>regclass</TT
>)</TT
></TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>Advance sequence and return new value</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>currval</CODE
>(<TT
CLASS="TYPE"
>regclass</TT
>)</TT
></TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>Return value most recently obtained with
<CODE
CLASS="FUNCTION"
>nextval</CODE
> for specified sequence</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>lastval</CODE
>()</TT
></TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>Return value most recently obtained with <CODE
CLASS="FUNCTION"
>nextval</CODE
></TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>setval</CODE
>(<TT
CLASS="TYPE"
>regclass</TT
>, <TT
CLASS="TYPE"
>bigint</TT
>)</TT
></TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>Set sequence's current value</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>setval</CODE
>(<TT
CLASS="TYPE"
>regclass</TT
>, <TT
CLASS="TYPE"
>bigint</TT
>, <TT
CLASS="TYPE"
>boolean</TT
>)</TT
></TD
><TD
><TT
CLASS="TYPE"
>bigint</TT
></TD
><TD
>Set sequence's current value and <TT
CLASS="LITERAL"
>is_called</TT
> flag</TD
></TR
></TBODY
></TABLE
></DIV
><P
> The sequence to be operated on by a sequence-function call is specified by
a <TT
CLASS="TYPE"
>regclass</TT
> argument, which is just the OID of the sequence in the
<TT
CLASS="STRUCTNAME"
>pg_class</TT
> system catalog. You do not have to look up the
OID by hand, however, since the <TT
CLASS="TYPE"
>regclass</TT
> 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. To
achieve some compatibility with the handling of ordinary
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> names, the string will be converted to lowercase
unless it contains double quotes around the sequence name. Thus
</P><PRE
CLASS="PROGRAMLISTING"
>nextval('foo') <I
CLASS="LINEANNOTATION"
>operates on sequence <TT
CLASS="LITERAL"
>foo</TT
></I
>
nextval('FOO') <I
CLASS="LINEANNOTATION"
>operates on sequence <TT
CLASS="LITERAL"
>foo</TT
></I
>
nextval('"Foo"') <I
CLASS="LINEANNOTATION"
>operates on sequence <TT
CLASS="LITERAL"
>Foo</TT
></I
></PRE
><P>
The sequence name can be schema-qualified if necessary:
</P><PRE
CLASS="PROGRAMLISTING"
>nextval('myschema.foo') <I
CLASS="LINEANNOTATION"
>operates on <TT
CLASS="LITERAL"
>myschema.foo</TT
></I
>
nextval('"myschema".foo') <I
CLASS="LINEANNOTATION"
>same as above</I
>
nextval('foo') <I
CLASS="LINEANNOTATION"
>searches search path for <TT
CLASS="LITERAL"
>foo</TT
></I
></PRE
><P>
See <A
HREF="datatype-oid.html"
>Section 8.12</A
> for more information about
<TT
CLASS="TYPE"
>regclass</TT
>.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Before <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.1, the arguments of the
sequence functions were of type <TT
CLASS="TYPE"
>text</TT
>, not <TT
CLASS="TYPE"
>regclass</TT
>, and
the above-described conversion from a text string to an OID value would
happen at run time during each call. For backwards compatibility, this
facility still exists, but internally it is now handled as an implicit
coercion from <TT
CLASS="TYPE"
>text</TT
> to <TT
CLASS="TYPE"
>regclass</TT
> before the function is
invoked.
</P
><P
> When you write the argument of a sequence function as an unadorned
literal string, it becomes a constant of type <TT
CLASS="TYPE"
>regclass</TT
>.
Since this is really just an OID, it will track the originally
identified sequence despite later renaming, schema reassignment,
etc. This <SPAN
CLASS="QUOTE"
>"early binding"</SPAN
> behavior is usually desirable for
sequence references in column defaults and views. But sometimes you will
want <SPAN
CLASS="QUOTE"
>"late binding"</SPAN
> where the sequence reference is resolved
at run time. To get late-binding behavior, force the constant to be
stored as a <TT
CLASS="TYPE"
>text</TT
> constant instead of <TT
CLASS="TYPE"
>regclass</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>nextval('foo'::text) <I
CLASS="LINEANNOTATION"
><TT
CLASS="LITERAL"
>foo</TT
> is looked up at runtime</I
></PRE
><P>
Note that late binding was the only behavior supported in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> releases before 8.1, so you
may need to do this to preserve the semantics of old applications.
</P
><P
> 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.
</P
></BLOCKQUOTE
></DIV
><P
> The available sequence functions are:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><CODE
CLASS="FUNCTION"
>nextval</CODE
></DT
><DD
><P
> Advance the sequence object to its next value and return that
value. This is done atomically: even if multiple sessions
execute <CODE
CLASS="FUNCTION"
>nextval</CODE
> concurrently, each will safely receive
a distinct sequence value.
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>currval</CODE
></DT
><DD
><P
> Return the value most recently obtained by <CODE
CLASS="FUNCTION"
>nextval</CODE
>
for this sequence in the current session. (An error is
reported if <CODE
CLASS="FUNCTION"
>nextval</CODE
> 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 whether or not
other sessions have executed <CODE
CLASS="FUNCTION"
>nextval</CODE
> since the
current session did.
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>lastval</CODE
></DT
><DD
><P
> Return the value most recently returned by
<CODE
CLASS="FUNCTION"
>nextval</CODE
> in the current session. This function is
identical to <CODE
CLASS="FUNCTION"
>currval</CODE
>, except that instead
of taking the sequence name as an argument it fetches the
value of the last sequence that <CODE
CLASS="FUNCTION"
>nextval</CODE
>
was used on in the current session. It is an error to call
<CODE
CLASS="FUNCTION"
>lastval</CODE
> if <CODE
CLASS="FUNCTION"
>nextval</CODE
>
has not yet been called in the current session.
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>setval</CODE
></DT
><DD
><P
> Reset the sequence object's counter value. The two-parameter
form sets the sequence's <TT
CLASS="LITERAL"
>last_value</TT
> field to the specified
value and sets its <TT
CLASS="LITERAL"
>is_called</TT
> field to <TT
CLASS="LITERAL"
>true</TT
>,
meaning that the next <CODE
CLASS="FUNCTION"
>nextval</CODE
> will advance the sequence
before returning a value. In the three-parameter form,
<TT
CLASS="LITERAL"
>is_called</TT
> may be set either <TT
CLASS="LITERAL"
>true</TT
> or
<TT
CLASS="LITERAL"
>false</TT
>. If it's set to <TT
CLASS="LITERAL"
>false</TT
>,
the next <CODE
CLASS="FUNCTION"
>nextval</CODE
> will return exactly the specified
value, and sequence advancement commences with the following
<CODE
CLASS="FUNCTION"
>nextval</CODE
>. For example,
</P><PRE
CLASS="SCREEN"
>SELECT setval('foo', 42); <I
CLASS="LINEANNOTATION"
>Next <CODE
CLASS="FUNCTION"
>nextval</CODE
> will return 43</I
>
SELECT setval('foo', 42, true); <I
CLASS="LINEANNOTATION"
>Same as above</I
>
SELECT setval('foo', 42, false); <I
CLASS="LINEANNOTATION"
>Next <CODE
CLASS="FUNCTION"
>nextval</CODE
> will return 42</I
></PRE
><P>
The result returned by <CODE
CLASS="FUNCTION"
>setval</CODE
> is just the value of its
second argument.
</P
></DD
></DL
></DIV
><P>
</P
><P
> If a sequence object has been created with default parameters,
<CODE
CLASS="FUNCTION"
>nextval</CODE
> calls on it will return successive values
beginning with 1. Other behaviors can be obtained by using
special parameters in the <A
HREF="sql-createsequence.html"
><I
>CREATE SEQUENCE</I
></A
> command;
see its command reference page for more information.
</P
><DIV
CLASS="IMPORTANT"
><BLOCKQUOTE
CLASS="IMPORTANT"
><P
><B
>Important: </B
> To avoid blocking of concurrent transactions that obtain numbers from the
same sequence, a <CODE
CLASS="FUNCTION"
>nextval</CODE
> operation is never rolled back;
that is, once a value has been fetched it is considered used, even if the
transaction that did the <CODE
CLASS="FUNCTION"
>nextval</CODE
> later aborts. This means
that aborted transactions may leave unused <SPAN
CLASS="QUOTE"
>"holes"</SPAN
> in the
sequence of assigned values. <CODE
CLASS="FUNCTION"
>setval</CODE
> operations are never
rolled back, either.
</P
></BLOCKQUOTE
></DIV
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