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diff --git a/doc/src/sgml/html/sql-createsequence.html b/doc/src/sgml/html/sql-createsequence.html new file mode 100644 index 0000000..5205ed0 --- /dev/null +++ b/doc/src/sgml/html/sql-createsequence.html @@ -0,0 +1,214 @@ +<?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>CREATE SEQUENCE</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="sql-createschema.html" title="CREATE SCHEMA" /><link rel="next" href="sql-createserver.html" title="CREATE SERVER" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CREATE SEQUENCE</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-createschema.html" title="CREATE SCHEMA">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-createserver.html" title="CREATE SERVER">Next</a></td></tr></table><hr /></div><div class="refentry" id="SQL-CREATESEQUENCE"><div class="titlepage"></div><a id="id-1.9.3.81.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CREATE SEQUENCE</span></h2><p>CREATE SEQUENCE — define a new sequence generator</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis"> +CREATE [ { TEMPORARY | TEMP } | UNLOGGED ] SEQUENCE [ IF NOT EXISTS ] <em class="replaceable"><code>name</code></em> + [ AS <em class="replaceable"><code>data_type</code></em> ] + [ INCREMENT [ BY ] <em class="replaceable"><code>increment</code></em> ] + [ MINVALUE <em class="replaceable"><code>minvalue</code></em> | NO MINVALUE ] [ MAXVALUE <em class="replaceable"><code>maxvalue</code></em> | NO MAXVALUE ] + [ START [ WITH ] <em class="replaceable"><code>start</code></em> ] [ CACHE <em class="replaceable"><code>cache</code></em> ] [ [ NO ] CYCLE ] + [ OWNED BY { <em class="replaceable"><code>table_name</code></em>.<em class="replaceable"><code>column_name</code></em> | NONE } ] +</pre></div><div class="refsect1" id="id-1.9.3.81.5"><h2>Description</h2><p> + <code class="command">CREATE SEQUENCE</code> creates a new sequence number + generator. This involves creating and initializing a new special + single-row table with the name <em class="replaceable"><code>name</code></em>. The generator will be + owned by the user issuing the command. + </p><p> + If a schema name is given then the sequence is created in the + specified schema. Otherwise it is created in the current schema. + Temporary sequences exist in a special schema, so a schema name cannot be + given when creating a temporary sequence. + The sequence name must be distinct from the name of any other relation + (table, sequence, index, view, materialized view, or foreign table) in + the same schema. + </p><p> + After a sequence is created, you use the functions + <code class="function">nextval</code>, + <code class="function">currval</code>, and + <code class="function">setval</code> + to operate on the sequence. These functions are documented in + <a class="xref" href="functions-sequence.html" title="9.17. Sequence Manipulation Functions">Section 9.17</a>. + </p><p> + Although you cannot update a sequence directly, you can use a query like: + +</p><pre class="programlisting"> +SELECT * FROM <em class="replaceable"><code>name</code></em>; +</pre><p> + + to examine the parameters and current state of a sequence. In particular, + the <code class="literal">last_value</code> field of the sequence shows the last value + allocated by any session. (Of course, this value might be obsolete + by the time it's printed, if other sessions are actively doing + <code class="function">nextval</code> calls.) + </p></div><div class="refsect1" id="id-1.9.3.81.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">TEMPORARY</code> or <code class="literal">TEMP</code></span></dt><dd><p> + If specified, the sequence object is created only for this + session, and is automatically dropped on session exit. Existing + permanent sequences with the same name are not visible (in this + session) while the temporary sequence exists, unless they are + referenced with schema-qualified names. + </p></dd><dt><span class="term"><code class="literal">UNLOGGED</code></span></dt><dd><p> + If specified, the sequence is created as an unlogged sequence. Changes + to unlogged sequences are not written to the write-ahead log. They are + not crash-safe: an unlogged sequence is automatically reset to its + initial state after a crash or unclean shutdown. Unlogged sequences are + also not replicated to standby servers. + </p><p> + Unlike unlogged tables, unlogged sequences do not offer a significant + performance advantage. This option is mainly intended for sequences + associated with unlogged tables via identity columns or serial columns. + In those cases, it usually wouldn't make sense to have the sequence + WAL-logged and replicated but not its associated table. + </p></dd><dt><span class="term"><code class="literal">IF NOT EXISTS</code></span></dt><dd><p> + Do not throw an error if a relation with the same name already exists. + A notice is issued in this case. Note that there is no guarantee that + the existing relation is anything like the sequence that would have + been created — it might not even be a sequence. + </p></dd><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p> + The name (optionally schema-qualified) of the sequence to be created. + </p></dd><dt><span class="term"><em class="replaceable"><code>data_type</code></em></span></dt><dd><p> + The optional + clause <code class="literal">AS <em class="replaceable"><code>data_type</code></em></code> + specifies the data type of the sequence. Valid types are + <code class="literal">smallint</code>, <code class="literal">integer</code>, + and <code class="literal">bigint</code>. <code class="literal">bigint</code> is the + default. The data type determines the default minimum and maximum + values of the sequence. + </p></dd><dt><span class="term"><em class="replaceable"><code>increment</code></em></span></dt><dd><p> + The optional clause <code class="literal">INCREMENT BY <em class="replaceable"><code>increment</code></em></code> specifies + which value is added to the current sequence value to create a + new value. A positive value will make an ascending sequence, a + negative one a descending sequence. The default value is 1. + </p></dd><dt><span class="term"><em class="replaceable"><code>minvalue</code></em><br /></span><span class="term"><code class="literal">NO MINVALUE</code></span></dt><dd><p> + The optional clause <code class="literal">MINVALUE <em class="replaceable"><code>minvalue</code></em></code> determines + the minimum value a sequence can generate. If this clause is not + supplied or <code class="option">NO MINVALUE</code> is specified, then + defaults will be used. The default for an ascending sequence is 1. The + default for a descending sequence is the minimum value of the data type. + </p></dd><dt><span class="term"><em class="replaceable"><code>maxvalue</code></em><br /></span><span class="term"><code class="literal">NO MAXVALUE</code></span></dt><dd><p> + The optional clause <code class="literal">MAXVALUE <em class="replaceable"><code>maxvalue</code></em></code> determines + the maximum value for the sequence. If this clause is not + supplied or <code class="option">NO MAXVALUE</code> is specified, then + default values will be used. The default for an ascending sequence is + the maximum value of the data type. The default for a descending + sequence is -1. + </p></dd><dt><span class="term"><em class="replaceable"><code>start</code></em></span></dt><dd><p> + The optional clause <code class="literal">START WITH <em class="replaceable"><code>start</code></em> </code> allows the + sequence to begin anywhere. The default starting value is + <em class="replaceable"><code>minvalue</code></em> for + ascending sequences and <em class="replaceable"><code>maxvalue</code></em> for descending ones. + </p></dd><dt><span class="term"><em class="replaceable"><code>cache</code></em></span></dt><dd><p> + The optional clause <code class="literal">CACHE <em class="replaceable"><code>cache</code></em></code> specifies how + many sequence numbers are to be preallocated and stored in + memory for faster access. The minimum value is 1 (only one value + can be generated at a time, i.e., no cache), and this is also the + default. + </p></dd><dt><span class="term"><code class="literal">CYCLE</code><br /></span><span class="term"><code class="literal">NO CYCLE</code></span></dt><dd><p> + The <code class="literal">CYCLE</code> option allows the sequence to wrap + around when the <em class="replaceable"><code>maxvalue</code></em> or <em class="replaceable"><code>minvalue</code></em> has been reached by an + ascending or descending sequence respectively. If the limit is + reached, the next number generated will be the <em class="replaceable"><code>minvalue</code></em> or <em class="replaceable"><code>maxvalue</code></em>, respectively. + </p><p> + If <code class="literal">NO CYCLE</code> is specified, any calls to + <code class="function">nextval</code> after the sequence has reached its + maximum value will return an error. If neither + <code class="literal">CYCLE</code> or <code class="literal">NO CYCLE</code> are + specified, <code class="literal">NO CYCLE</code> is the default. + </p></dd><dt><span class="term"><code class="literal">OWNED BY</code> <em class="replaceable"><code>table_name</code></em>.<em class="replaceable"><code>column_name</code></em><br /></span><span class="term"><code class="literal">OWNED BY NONE</code></span></dt><dd><p> + The <code class="literal">OWNED BY</code> option causes the sequence to be + associated with a specific table column, such that if that column + (or its whole table) is dropped, the sequence will be automatically + dropped as well. The specified table must have the same owner and be in + the same schema as the sequence. + <code class="literal">OWNED BY NONE</code>, the default, specifies that there + is no such association. + </p></dd></dl></div></div><div class="refsect1" id="id-1.9.3.81.7"><h2>Notes</h2><p> + Use <code class="command">DROP SEQUENCE</code> to remove a sequence. + </p><p> + Sequences are based on <code class="type">bigint</code> arithmetic, so the range + cannot exceed the range of an eight-byte integer + (-9223372036854775808 to 9223372036854775807). + </p><p> + Because <code class="function">nextval</code> and <code class="function">setval</code> calls are never + rolled back, sequence objects cannot be used if <span class="quote">“<span class="quote">gapless</span>”</span> + assignment of sequence numbers is needed. It is possible to build + gapless assignment by using exclusive locking of a table containing a + counter; but this solution is much more expensive than sequence + objects, especially if many transactions need sequence numbers + concurrently. + </p><p> + Unexpected results might be obtained if a <em class="replaceable"><code>cache</code></em> setting greater than one is + used for a sequence object that will be used concurrently by + multiple sessions. Each session will allocate and cache successive + sequence values during one access to the sequence object and + increase the sequence object's <code class="literal">last_value</code> accordingly. + Then, the next <em class="replaceable"><code>cache</code></em>-1 + uses of <code class="function">nextval</code> within that session simply return the + preallocated values without touching the sequence object. So, any + numbers allocated but not used within a session will be lost when + that session ends, resulting in <span class="quote">“<span class="quote">holes</span>”</span> in the + sequence. + </p><p> + Furthermore, although multiple sessions are guaranteed to allocate + distinct sequence values, the values might be generated out of + sequence when all the sessions are considered. For example, with + a <em class="replaceable"><code>cache</code></em> setting of 10, + session A might reserve values 1..10 and return + <code class="function">nextval</code>=1, then session B might reserve values + 11..20 and return <code class="function">nextval</code>=11 before session A + has generated <code class="function">nextval</code>=2. Thus, with a + <em class="replaceable"><code>cache</code></em> setting of one + it is safe to assume that <code class="function">nextval</code> values are generated + sequentially; with a <em class="replaceable"><code>cache</code></em> setting greater than one you + should only assume that the <code class="function">nextval</code> values are all + distinct, not that they are generated purely sequentially. Also, + <code class="literal">last_value</code> will reflect the latest value reserved by + any session, whether or not it has yet been returned by + <code class="function">nextval</code>. + </p><p> + Another consideration is that a <code class="function">setval</code> executed on + such a sequence will not be noticed by other sessions until they + have used up any preallocated values they have cached. + </p></div><div class="refsect1" id="id-1.9.3.81.8"><h2>Examples</h2><p> + Create an ascending sequence called <code class="literal">serial</code>, starting at 101: +</p><pre class="programlisting"> +CREATE SEQUENCE serial START 101; +</pre><p> + </p><p> + Select the next number from this sequence: +</p><pre class="programlisting"> +SELECT nextval('serial'); + + nextval +--------- + 101 +</pre><p> + </p><p> + Select the next number from this sequence: +</p><pre class="programlisting"> +SELECT nextval('serial'); + + nextval +--------- + 102 +</pre><p> + </p><p> + Use this sequence in an <code class="command">INSERT</code> command: +</p><pre class="programlisting"> +INSERT INTO distributors VALUES (nextval('serial'), 'nothing'); +</pre><p> + </p><p> + Update the sequence value after a <code class="command">COPY FROM</code>: +</p><pre class="programlisting"> +BEGIN; +COPY distributors FROM 'input_file'; +SELECT setval('serial', max(id)) FROM distributors; +END; +</pre></div><div class="refsect1" id="id-1.9.3.81.9"><h2>Compatibility</h2><p> + <code class="command">CREATE SEQUENCE</code> conforms to the <acronym class="acronym">SQL</acronym> + standard, with the following exceptions: + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + Obtaining the next value is done using the <code class="function">nextval()</code> + function instead of the standard's <code class="command">NEXT VALUE FOR</code> + expression. + </p></li><li class="listitem"><p> + The <code class="literal">OWNED BY</code> clause is a <span class="productname">PostgreSQL</span> + extension. + </p></li></ul></div></div><div class="refsect1" id="id-1.9.3.81.10"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-altersequence.html" title="ALTER SEQUENCE"><span class="refentrytitle">ALTER SEQUENCE</span></a>, <a class="xref" href="sql-dropsequence.html" title="DROP SEQUENCE"><span class="refentrytitle">DROP SEQUENCE</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-createschema.html" title="CREATE SCHEMA">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createserver.html" title="CREATE SERVER">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CREATE SCHEMA </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE SERVER</td></tr></table></div></body></html>
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