<?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>9.16. 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="9.15. 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 11.4 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="8.19. Object Identifier Types">Section 8.19</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><div class="important"><h3 class="title">Important</h3><p> 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 and will not be returned again. This is true even if the surrounding transaction later aborts, or if the calling query ends up not using the value. 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. Such cases will leave unused <span class="quote">“<span class="quote">holes</span>”</span> in the sequence of assigned values. 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></div><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><div class="important"><h3 class="title">Important</h3><p> Because sequences are non-transactional, changes made by <code class="function">setval</code> are not undone if the transaction rolls back. </p></div><p> This function requires <code class="literal">UPDATE</code> privilege on the sequence. </p></dd></dl></div><p> </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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="functions.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="functions-conditional.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">9.15. JSON Functions and Operators </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 9.17. Conditional Expressions</td></tr></table></div></body></html>