summaryrefslogtreecommitdiffstats
path: root/doc/src/sgml/html/sql-createsequence.html
blob: 8be1417f97dbe2402899e73c4eca275a9d24b391 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
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 15.7 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 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE SERVER</td></tr></table></div></body></html>