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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>9.16. Sequence Manipulation Functions</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="[email protected]" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="functions-json.html" title="9.15. JSON Functions and Operators" /><link rel="next" href="functions-conditional.html" title="9.17. Conditional Expressions" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">9.16. Sequence Manipulation Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="functions-json.html" title="9.15. JSON Functions and Operators">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="functions.html" title="Chapter 9. Functions and Operators">Up</a></td><th width="60%" align="center">Chapter 9. Functions and Operators</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 10.23 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="functions-conditional.html" title="9.17. Conditional Expressions">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="FUNCTIONS-SEQUENCE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">9.16. Sequence Manipulation Functions</h2></div></div></div><a id="id-1.5.8.21.2" class="indexterm"></a><a id="id-1.5.8.21.3" class="indexterm"></a><a id="id-1.5.8.21.4" class="indexterm"></a><a id="id-1.5.8.21.5" class="indexterm"></a><a id="id-1.5.8.21.6" class="indexterm"></a><p> This section describes functions for operating on <em class="firstterm">sequence
objects</em>, also called sequence generators or just sequences.
Sequence objects are special single-row tables created with <a class="xref" href="sql-createsequence.html" title="CREATE SEQUENCE"><span class="refentrytitle">CREATE SEQUENCE</span></a>.
Sequence objects are commonly used to generate unique identifiers
for rows of a table. The sequence functions, listed in <a class="xref" href="functions-sequence.html#FUNCTIONS-SEQUENCE-TABLE" title="Table 9.47. Sequence Functions">Table 9.47</a>, provide simple, multiuser-safe
methods for obtaining successive sequence values from sequence
objects.
</p><div class="table" id="FUNCTIONS-SEQUENCE-TABLE"><p class="title"><strong>Table 9.47. Sequence Functions</strong></p><div class="table-contents"><table class="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">currval(<code class="type">regclass</code>)</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> for any sequence</td></tr><tr><td><code class="literal"><code class="function">nextval(<code class="type">regclass</code>)</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">setval(<code class="type">regclass</code>, <code class="type">bigint</code>)</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 class="type">regclass</code>, <code class="type">bigint</code>, <code class="type">boolean</code>)</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 is specified by
a <code class="type">regclass</code> argument, which is simply 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. 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') <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 class="xref" href="datatype-oid.html" title="8.18. Object Identifier Types">Section 8.18</a> for more information about
<code class="type">regclass</code>.
</p><div class="note"><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 backward 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">“<span class="quote">early binding</span>”</span> behavior is usually desirable for
sequence references in column defaults and views. But sometimes you might
want <span class="quote">“<span class="quote">late binding</span>”</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
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></div><p> The available sequence functions are:
</p><div class="variablelist"><dl class="variablelist"><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><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 class="xref" href="sql-createsequence.html" title="CREATE SEQUENCE"><span class="refentrytitle">CREATE SEQUENCE</span></a> command;
see its command reference page for more information.
</p><p> This function requires <code class="literal">USAGE</code>
or <code class="literal">UPDATE</code> privilege on the sequence.
</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.) 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><p> This function requires <code class="literal">USAGE</code>
or <code class="literal">SELECT</code> privilege on the sequence.
</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 refers to whichever
sequence <code class="function">nextval</code> was most recently applied to
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><p> This function requires <code class="literal">USAGE</code>
or <code class="literal">SELECT</code> privilege on the last used sequence.
</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. The value reported by <code class="function">currval</code> is
also set to the specified value. In the three-parameter form,
<code class="literal">is_called</code> can be set to either <code class="literal">true</code>
or <code class="literal">false</code>. <code class="literal">true</code> has the same effect as
the two-parameter form. If it is 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>. Furthermore, the value reported by
<code class="function">currval</code> is not changed in this case. 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><p> This function requires <code class="literal">UPDATE</code> privilege on the
sequence.
</p></dd></dl></div><p>
</p><div class="caution"><h3 class="title">Caution</h3><p> To avoid blocking concurrent transactions that obtain numbers from
the same sequence, the value obtained by <code class="function">nextval</code>
is not reclaimed for re-use if the calling transaction later aborts.
This means that transaction aborts or database crashes can result in
gaps in the sequence of assigned values. That can happen without a
transaction abort, too. For example an <code class="command">INSERT</code> with
an <code class="literal">ON CONFLICT</code> clause will compute the to-be-inserted
tuple, including doing any required <code class="function">nextval</code>
calls, before detecting any conflict that would cause it to follow
the <code class="literal">ON CONFLICT</code> rule instead.
Thus, <span class="productname">PostgreSQL</span> sequence
objects <span class="emphasis"><em>cannot be used to obtain <span class="quote">“<span class="quote">gapless</span>”</span>
sequences</em></span>.
</p><p> Likewise, sequence state changes made by <code class="function">setval</code>
are immediately visible to other transactions, and are not undone if
the calling transaction rolls back.
</p><p> If the database cluster crashes before committing a transaction
containing a <code class="function">nextval</code>
or <code class="function">setval</code> call, the sequence state change might
not have made its way to persistent storage, so that it is uncertain
whether the sequence will have its original or updated state after the
cluster restarts. This is harmless for usage of the sequence within
the database, since other effects of uncommitted transactions will not
be visible either. However, if you wish to use a sequence value for
persistent outside-the-database purposes, make sure that the
<code class="function">nextval</code> call has been committed before doing so.
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