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>Sequence Manipulation Functions</TITLE
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><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-SEQUENCE"
>9.16. Sequence Manipulation Functions</A
></H1
><P
> This section describes functions for operating on <I
CLASS="FIRSTTERM"
>sequence
objects</I
>, also called sequence generators or just sequences.
Sequence objects are special single-row tables created with <A
HREF="sql-createsequence.html"
>CREATE SEQUENCE</A
>.
Sequence objects are commonly used to generate unique identifiers
for rows of a table. The sequence functions, listed in <A
HREF="functions-sequence.html#FUNCTIONS-SEQUENCE-TABLE"
>Table 9-41</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 9-41. 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"
>currval(<TT
CLASS="TYPE"
>regclass</TT
>)</CODE
></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
> for any sequence</TD
></TR
><TR
><TD
><TT
CLASS="LITERAL"
><CODE
CLASS="FUNCTION"
>nextval(<TT
CLASS="TYPE"
>regclass</TT
>)</CODE
></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"
>setval(<TT
CLASS="TYPE"
>regclass</TT
>, <TT
CLASS="TYPE"
>bigint</TT
>)</CODE
></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(<TT
CLASS="TYPE"
>regclass</TT
>, <TT
CLASS="TYPE"
>bigint</TT
>, <TT
CLASS="TYPE"
>boolean</TT
>)</CODE
></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 is specified by
a <TT
CLASS="TYPE"
>regclass</TT
> argument, which is simply 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. For
compatibility with the handling of ordinary
<ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> names, the string will be converted to lower case
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 8.18</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 backward 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 might
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
might 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
><P
> If a sequence object has been created with default parameters,
successive <CODE
CLASS="FUNCTION"
>nextval</CODE
> calls will return successive
values beginning with 1. Other behaviors can be obtained by using
special parameters in the <A
HREF="sql-createsequence.html"
>CREATE SEQUENCE</A
> command;
see its command reference page for more information.
</P
><DIV
CLASS="IMPORTANT"
><BLOCKQUOTE
CLASS="IMPORTANT"
><P
><B
>Important: </B
> To avoid blocking 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 might leave unused <SPAN
CLASS="QUOTE"
>"holes"</SPAN
> in the sequence
of assigned values.
</P
></BLOCKQUOTE
></DIV
></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.) 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 used by <CODE
CLASS="FUNCTION"
>nextval</CODE
>
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. The value reported by <CODE
CLASS="FUNCTION"
>currval</CODE
> is
also set to the specified value. In the three-parameter form,
<TT
CLASS="LITERAL"
>is_called</TT
> can be set to either <TT
CLASS="LITERAL"
>true</TT
>
or <TT
CLASS="LITERAL"
>false</TT
>. <TT
CLASS="LITERAL"
>true</TT
> has the same effect as
the two-parameter form. If it is 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
>. Furthermore, the value reported by
<CODE
CLASS="FUNCTION"
>currval</CODE
> is not changed in this case (this is a change
from pre-8.3 behavior). 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
><DIV
CLASS="IMPORTANT"
><BLOCKQUOTE
CLASS="IMPORTANT"
><P
><B
>Important: </B
> Because sequences are non-transactional, changes made by
<CODE
CLASS="FUNCTION"
>setval</CODE
> are not undone if the transaction rolls
back.
</P
></BLOCKQUOTE
></DIV
></DD
></DL
></DIV
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