Baoding-FenShuaJiang
/
Green_GoodERP18_FSJ_Standard
关注 1
点赞 0
百科 捐赠
gooderp18绿色标准版
您最多选择25个主题 主题必须以字母或数字开头，可以包含连字符 (-)，并且长度不得超过35个字符
分支: master
分支列表 标签列表
${ item.name }
创建分支 ${ searchTerm }
从 'master'
${ noResults }
Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/functions-sequence.html

149 行
15KB
原始文件 永久链接 Blame 文件历史 

  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!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 V1.79.1" /><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 12.4 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>

  3.    This section describes functions for operating on <em class="firstterm">sequence

  4.    objects</em>, also called sequence generators or just sequences.

  5.    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>.

  6.    Sequence objects are commonly used to generate unique identifiers

  7.    for rows of a table.  The sequence functions, listed in <a class="xref" href="functions-sequence.html#FUNCTIONS-SEQUENCE-TABLE" title="Table 9.50. Sequence Functions">Table 9.50</a>, provide simple, multiuser-safe

  8.    methods for obtaining successive sequence values from sequence

  9.    objects.

  10.   </p><div class="table" id="FUNCTIONS-SEQUENCE-TABLE"><p class="title"><strong>Table 9.50. 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

  11.         <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

  12.         <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>

  13.    The sequence to be operated on by a sequence function is specified by

  14.    a <code class="type">regclass</code> argument, which is simply the OID of the sequence in the

  15.    <code class="structname">pg_class</code> system catalog.  You do not have to look up the

  16.    OID by hand, however, since the <code class="type">regclass</code> data type's input

  17.    converter will do the work for you.  Just write the sequence name enclosed

  18.    in single quotes so that it looks like a literal constant.  For

  19.    compatibility with the handling of ordinary

  20.    <acronym class="acronym">SQL</acronym> names, the string will be converted to lower case

  21.    unless it contains double quotes around the sequence name.  Thus:

  22. </p><pre class="programlisting">

  23. nextval('foo')      <em class="lineannotation"><span class="lineannotation">operates on sequence <code class="literal">foo</code></span></em>

  24. nextval('FOO')      <em class="lineannotation"><span class="lineannotation">operates on sequence <code class="literal">foo</code></span></em>

  25. nextval('"Foo"')    <em class="lineannotation"><span class="lineannotation">operates on sequence <code class="literal">Foo</code></span></em>

  26. </pre><p>

  27.    The sequence name can be schema-qualified if necessary:

  28. </p><pre class="programlisting">

  29. nextval('myschema.foo')     <em class="lineannotation"><span class="lineannotation">operates on <code class="literal">myschema.foo</code></span></em>

  30. nextval('"myschema".foo')   <em class="lineannotation"><span class="lineannotation">same as above</span></em>

  31. nextval('foo')              <em class="lineannotation"><span class="lineannotation">searches search path for <code class="literal">foo</code></span></em>

  32. </pre><p>

  33.    See <a class="xref" href="datatype-oid.html" title="8.19. Object Identifier Types">Section 8.19</a> for more information about

  34.    <code class="type">regclass</code>.

  35.   </p><div class="note"><h3 class="title">Note</h3><p>

  36.     Before <span class="productname">PostgreSQL</span> 8.1, the arguments of the

  37.     sequence functions were of type <code class="type">text</code>, not <code class="type">regclass</code>, and

  38.     the above-described conversion from a text string to an OID value would

  39.     happen at run time during each call.  For backward compatibility, this

  40.     facility still exists, but internally it is now handled as an implicit

  41.     coercion from <code class="type">text</code> to <code class="type">regclass</code> before the function is

  42.     invoked.

  43.    </p><p>

  44.     When you write the argument of a sequence function as an unadorned

  45.     literal string, it becomes a constant of type <code class="type">regclass</code>.

  46.     Since this is really just an OID, it will track the originally

  47.     identified sequence despite later renaming, schema reassignment,

  48.     etc.  This <span class="quote">“<span class="quote">early binding</span>”</span> behavior is usually desirable for

  49.     sequence references in column defaults and views.  But sometimes you might

  50.     want <span class="quote">“<span class="quote">late binding</span>”</span> where the sequence reference is resolved

  51.     at run time.  To get late-binding behavior, force the constant to be

  52.     stored as a <code class="type">text</code> constant instead of <code class="type">regclass</code>:

  53. </p><pre class="programlisting">

  54. nextval('foo'::text)      <em class="lineannotation"><span class="lineannotation"><code class="literal">foo</code> is looked up at runtime</span></em>

  55. </pre><p>

  56.     Note that late binding was the only behavior supported in

  57.     <span class="productname">PostgreSQL</span> releases before 8.1, so you

  58.     might need to do this to preserve the semantics of old applications.

  59.    </p><p>

  60.     Of course, the argument of a sequence function can be an expression

  61.     as well as a constant.  If it is a text expression then the implicit

  62.     coercion will result in a run-time lookup.

  63.    </p></div><p>

  64.    The available sequence functions are:

  65. 

  66.     </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="function">nextval</code></span></dt><dd><p>

  67.         Advance the sequence object to its next value and return that

  68.         value.  This is done atomically: even if multiple sessions

  69.         execute <code class="function">nextval</code> concurrently, each will safely receive

  70.         a distinct sequence value.

  71.        </p><p>

  72.         If a sequence object has been created with default parameters,

  73.         successive <code class="function">nextval</code> calls will return successive

  74.         values beginning with 1.  Other behaviors can be obtained by using

  75.         special parameters in the <a class="xref" href="sql-createsequence.html" title="CREATE SEQUENCE"><span class="refentrytitle">CREATE SEQUENCE</span></a> command;

  76.         see its command reference page for more information.

  77.        </p><div class="important"><h3 class="title">Important</h3><p>

  78.          To avoid blocking concurrent transactions that obtain numbers from

  79.          the same sequence, a <code class="function">nextval</code> operation is never

  80.          rolled back; that is, once a value has been fetched it is considered

  81.          used and will not be returned again.  This is true even if the

  82.          surrounding transaction later aborts, or if the calling query ends

  83.          up not using the value.  For example an <code class="command">INSERT</code> with

  84.          an <code class="literal">ON CONFLICT</code> clause will compute the to-be-inserted

  85.          tuple, including doing any required <code class="function">nextval</code>

  86.          calls, before detecting any conflict that would cause it to follow

  87.          the <code class="literal">ON CONFLICT</code> rule instead.  Such cases will leave

  88.          unused <span class="quote">“<span class="quote">holes</span>”</span> in the sequence of assigned values.

  89.          Thus, <span class="productname">PostgreSQL</span> sequence objects <span class="emphasis"><em>cannot

  90.          be used to obtain <span class="quote">“<span class="quote">gapless</span>”</span> sequences</em></span>.

  91.         </p></div><p>

  92.         This function requires <code class="literal">USAGE</code>

  93.         or <code class="literal">UPDATE</code> privilege on the sequence.

  94.        </p></dd><dt><span class="term"><code class="function">currval</code></span></dt><dd><p>

  95.         Return the value most recently obtained by <code class="function">nextval</code>

  96.         for this sequence in the current session.  (An error is

  97.         reported if <code class="function">nextval</code> has never been called for this

  98.         sequence in this session.)  Because this is returning

  99.         a session-local value, it gives a predictable answer whether or not

  100.         other sessions have executed <code class="function">nextval</code> since the

  101.         current session did.

  102.        </p><p>

  103.         This function requires <code class="literal">USAGE</code>

  104.         or <code class="literal">SELECT</code> privilege on the sequence.

  105.        </p></dd><dt><span class="term"><code class="function">lastval</code></span></dt><dd><p>

  106.         Return the value most recently returned by

  107.         <code class="function">nextval</code> in the current session. This function is

  108.         identical to <code class="function">currval</code>, except that instead

  109.         of taking the sequence name as an argument it refers to whichever

  110.         sequence <code class="function">nextval</code> was most recently applied to

  111.         in the current session. It is an error to call

  112.         <code class="function">lastval</code> if <code class="function">nextval</code>

  113.         has not yet been called in the current session.

  114.        </p><p>

  115.         This function requires <code class="literal">USAGE</code>

  116.         or <code class="literal">SELECT</code> privilege on the last used sequence.

  117.        </p></dd><dt><span class="term"><code class="function">setval</code></span></dt><dd><p>

  118.         Reset the sequence object's counter value.  The two-parameter

  119.         form sets the sequence's <code class="literal">last_value</code> field to the

  120.         specified value and sets its <code class="literal">is_called</code> field to

  121.         <code class="literal">true</code>, meaning that the next

  122.         <code class="function">nextval</code> will advance the sequence before

  123.         returning a value.  The value reported by <code class="function">currval</code> is

  124.         also set to the specified value.  In the three-parameter form,

  125.         <code class="literal">is_called</code> can be set to either <code class="literal">true</code>

  126.         or <code class="literal">false</code>.  <code class="literal">true</code> has the same effect as

  127.         the two-parameter form. If it is set to <code class="literal">false</code>, the

  128.         next <code class="function">nextval</code> will return exactly the specified

  129.         value, and sequence advancement commences with the following

  130.         <code class="function">nextval</code>.  Furthermore, the value reported by

  131.         <code class="function">currval</code> is not changed in this case.  For example,

  132. 

  133. </p><pre class="screen">

  134. SELECT setval('foo', 42);           <em class="lineannotation"><span class="lineannotation">Next <code class="function">nextval</code> will return 43</span></em>

  135. SELECT setval('foo', 42, true);     <em class="lineannotation"><span class="lineannotation">Same as above</span></em>

  136. SELECT setval('foo', 42, false);    <em class="lineannotation"><span class="lineannotation">Next <code class="function">nextval</code> will return 42</span></em>

  137. </pre><p>

  138. 

  139.         The result returned by <code class="function">setval</code> is just the value of its

  140.         second argument.

  141.        </p><div class="important"><h3 class="title">Important</h3><p>

  142.          Because sequences are non-transactional, changes made by

  143.          <code class="function">setval</code> are not undone if the transaction rolls

  144.          back.

  145.         </p></div><p>

  146.         This function requires <code class="literal">UPDATE</code> privilege on the

  147.         sequence.

  148.        </p></dd></dl></div><p>

  149.   </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>
上海开阖软件有限公司 沪ICP备12045867号-1