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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!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>9.17. Sequence Manipulation Functions</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="functions-json.html" title="9.16. JSON Functions and Operators" /><link rel="next" href="functions-conditional.html" title="9.18. Conditional Expressions" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">9.17. Sequence Manipulation Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="functions-json.html" title="9.16. 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 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="functions-conditional.html" title="9.18. Conditional Expressions">Next</a></td></tr></table><hr /></div><div class="sect1" id="FUNCTIONS-SEQUENCE"><div class="titlepage"><div><div><h2 class="title" style="clear: both">9.17. Sequence Manipulation Functions</h2></div></div></div><a id="id-1.5.8.23.2" 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 9.51. Sequence Functions">Table 9.51</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 9.51. Sequence Functions</strong></p><div class="table-contents"><table class="table" summary="Sequence Functions" border="1"><colgroup><col /></colgroup><thead><tr><th class="func_table_entry"><p class="func_signature">
Function
</p>
<p>
Description
</p></th></tr></thead><tbody><tr><td class="func_table_entry"><p class="func_signature">
<a id="id-1.5.8.23.4.2.2.1.1.1.1" class="indexterm"></a>
<code class="function">nextval</code> ( <code class="type">regclass</code> )
→ <code class="returnvalue">bigint</code>
</p>
<p>
Advances the sequence object to its next value and returns 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.
If the 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
appropriate parameters in the <a class="xref" href="sql-createsequence.html" title="CREATE SEQUENCE"><span class="refentrytitle">CREATE SEQUENCE</span></a>
command.
</p>
<p>
This function requires <code class="literal">USAGE</code>
or <code class="literal">UPDATE</code> privilege on the sequence.
</p></td></tr><tr><td class="func_table_entry"><p class="func_signature">
<a id="id-1.5.8.23.4.2.2.2.1.1.1" class="indexterm"></a>
<code class="function">setval</code> ( <code class="type">regclass</code>, <code class="type">bigint</code> [<span class="optional">, <code class="type">boolean</code> </span>] )
→ <code class="returnvalue">bigint</code>
</p>
<p>
Sets the sequence object's current value, and optionally
its <code class="literal">is_called</code> flag. 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 that will be 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="programlisting">
SELECT setval('myseq', 42); <em class="lineannotation"><span class="lineannotation">Next <code class="function">nextval</code> will return 43</span></em>
SELECT setval('myseq', 42, true); <em class="lineannotation"><span class="lineannotation">Same as above</span></em>
SELECT setval('myseq', 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></td></tr><tr><td class="func_table_entry"><p class="func_signature">
<a id="id-1.5.8.23.4.2.2.3.1.1.1" class="indexterm"></a>
<code class="function">currval</code> ( <code class="type">regclass</code> )
→ <code class="returnvalue">bigint</code>
</p>
<p>
Returns 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></td></tr><tr><td class="func_table_entry"><p class="func_signature">
<a id="id-1.5.8.23.4.2.2.4.1.1.1" class="indexterm"></a>
<code class="function">lastval</code> ()
→ <code class="returnvalue">bigint</code>
</p>
<p>
Returns 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></td></tr></tbody></table></div></div><br class="table-break" /><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.
</p></div><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. See <a class="xref" href="datatype-oid.html" title="8.19. Object Identifier Types">Section 8.19</a>
for details.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="functions-json.html" title="9.16. JSON Functions and Operators">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="functions.html" title="Chapter 9. Functions and Operators">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="functions-conditional.html" title="9.18. Conditional Expressions">Next</a></td></tr><tr><td width="40%" align="left" valign="top">9.16. JSON Functions and Operators </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 9.18. Conditional Expressions</td></tr></table></div></body></html>
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