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<title>9.12.Sequence Manipulation Functions</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="functions-sequence"></a>9.12.Sequence Manipulation Functions</h2></div></div></div>
<a name="id616909"></a><a name="id616916"></a><a name="id616922"></a><a name="id616928"></a><a name="id616934"></a><p> This section describes <span class="productname">PostgreSQL</span>'s functions
for operating on <em class="firstterm">sequence objects</em>.
Sequence objects (also called sequence generators or
just sequences) are special single-row tables created with
<code class="command">CREATE SEQUENCE</code>. 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" title="Table9.34.Sequence Functions">Table9.34, “Sequence Functions”</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 class="title"><b>Table9.34.Sequence Functions</b></p>
<div class="table-contents"><table summary="Sequence Functions" border="1">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>Function</th>
<th>Return Type</th>
<th>Description</th>
</tr></thead>
<tbody>
<tr>
<td><code class="literal"><code class="function">nextval</code>(<code class="type">regclass</code>)</code></td>
<td><code class="type">bigint</code></td>
<td>Advance sequence and return new value</td>
</tr>
<tr>
<td><code class="literal"><code class="function">currval</code>(<code class="type">regclass</code>)</code></td>
<td><code class="type">bigint</code></td>
<td>Return value most recently obtained with
<code class="function">nextval</code> for specified sequence</td>
</tr>
<tr>
<td><code class="literal"><code class="function">lastval</code>()</code></td>
<td><code class="type">bigint</code></td>
<td>Return value most recently obtained with <code class="function">nextval</code>
</td>
</tr>
<tr>
<td><code class="literal"><code class="function">setval</code>(<code class="type">regclass</code>, <code class="type">bigint</code>)</code></td>
<td><code class="type">bigint</code></td>
<td>Set sequence's current value</td>
</tr>
<tr>
<td><code class="literal"><code class="function">setval</code>(<code class="type">regclass</code>, <code class="type">bigint</code>, <code class="type">boolean</code>)</code></td>
<td><code class="type">bigint</code></td>
<td>Set sequence's current value and <code class="literal">is_called</code> flag</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p> The sequence to be operated on by a sequence-function call is specified by
a <code class="type">regclass</code> argument, which is just the OID of the sequence in the
<code class="structname">pg_class</code> system catalog. You do not have to look up the
OID by hand, however, since the <code class="type">regclass</code> 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') <em class="lineannotation"><span class="lineannotation">operates on sequence <code class="literal">foo</code></span></em>
nextval('FOO') <em class="lineannotation"><span class="lineannotation">operates on sequence <code class="literal">foo</code></span></em>
nextval('"Foo"') <em class="lineannotation"><span class="lineannotation">operates on sequence <code class="literal">Foo</code></span></em></pre>
<p>
The sequence name can be schema-qualified if necessary:
</p>
<pre class="programlisting">nextval('myschema.foo') <em class="lineannotation"><span class="lineannotation">operates on <code class="literal">myschema.foo</code></span></em>
nextval('"myschema".foo') <em class="lineannotation"><span class="lineannotation">same as above</span></em>
nextval('foo') <em class="lineannotation"><span class="lineannotation">searches search path for <code class="literal">foo</code></span></em></pre>
<p>
See <a href="datatype-oid.html" title="8.12.Object Identifier Types">Section8.12, “Object Identifier Types”</a> for more information about
<code class="type">regclass</code>.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p> Before <span class="productname">PostgreSQL</span> 8.1, the arguments of the
sequence functions were of type <code class="type">text</code>, not <code class="type">regclass</code>, 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 <code class="type">text</code> to <code class="type">regclass</code> 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 <code class="type">regclass</code>.
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 <code class="type">text</code> constant instead of <code class="type">regclass</code>:
</p>
<pre class="programlisting">nextval('foo'::text) <em class="lineannotation"><span class="lineannotation"><code class="literal">foo</code> is looked up at runtime</span></em></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>
</div>
<p> The available sequence functions are:
</p>
<div class="variablelist"><dl>
<dt><span class="term"><code class="function">nextval</code></span></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><span class="term"><code class="function">currval</code></span></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><span class="term"><code class="function">lastval</code></span></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><span class="term"><code class="function">setval</code></span></dt>
<dd>
<p> Reset the sequence object's counter value. The two-parameter
form sets the sequence's <code class="literal">last_value</code> field to the specified
value and sets its <code class="literal">is_called</code> field to <code class="literal">true</code>,
meaning that the next <code class="function">nextval</code> will advance the sequence
before returning a value. In the three-parameter form,
<code class="literal">is_called</code> may be set either <code class="literal">true</code> or
<code class="literal">false</code>. If it's set to <code class="literal">false</code>,
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); <em class="lineannotation"><span class="lineannotation">Next <code class="function">nextval</code> will return 43</span></em>
SELECT setval('foo', 42, true); <em class="lineannotation"><span class="lineannotation">Same as above</span></em>
SELECT setval('foo', 42, false); <em class="lineannotation"><span class="lineannotation">Next <code class="function">nextval</code> will return 42</span></em></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">CREATE SEQUENCE</a> command;
see its command reference page for more information.
</p>
<div class="important" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Important</h3>
<p> 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>
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