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Expand documentation for sequence functions (nextval and friends).

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Place it in the expected place in the User's Guide, rather than
hiding it in the command reference page for CREATE SEQUENCE.
REL7_2_STABLE
Tom Lane 24 years ago
parent 59fdccccf5
commit 2043340b87
3 changed files with 203 additions and 64 deletions
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  1. 165
      doc/src/sgml/func.sgml
  2. 96
      doc/src/sgml/ref/create_sequence.sgml
  3. 6
      doc/src/sgml/ref/grant.sgml

165
doc/src/sgml/func.sgml
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@ -1,4 +1,7 @@
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.78 2001/10/12 02:08:34 ishii Exp $ -->
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.79 2001/11/17 22:20:33 tgl Exp $
Postgres documentation
-->
<chapter id="functions">
<title>Functions and Operators</title>
@ -3627,6 +3630,166 @@ SELECT TIMESTAMP 'now';
</sect1>
<sect1 id="functions-sequence">
<title>Sequence-Manipulation Functions</title>
<indexterm>
<primary>sequences</primary>
</indexterm>
<indexterm>
<primary>nextval</primary>
</indexterm>
<indexterm>
<primary>currval</primary>
</indexterm>
<indexterm>
<primary>setval</primary>
</indexterm>
<table>
<title>Sequence Functions</>
<tgroup cols="3">
<thead>
<row><entry>Function</> <entry>Returns</> <entry>Description</></row>
</thead>
<tbody>
<row>
<entry><function>nextval</function>(<type>text</type>)</entry>
<entry><type>bigint</type></entry>
<entry>Advance sequence and return new value</>
</row>
<row>
<entry><function>currval</function>(<type>text</type>)</entry>
<entry><type>bigint</type></entry>
<entry>Return value most recently obtained with <function>nextval</></entry>
</row>
<row>
<entry><function>setval</function>(<type>text</type>,<type>bigint</type>)</entry>
<entry><type>bigint</type></entry>
<entry>Set sequence's current value</>
</row>
<row>
<entry><function>setval</function>(<type>text</type>,<type>bigint</type>,<type>boolean</>)</entry>
<entry><type>bigint</type></entry>
<entry>Set sequence's current value and <literal>is_called</> flag</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
This section describes <productname>Postgres</productname>'s functions
for operating on <firstterm>sequence objects</>.
Sequence objects (also called sequence generators or
just sequences) are special single-row tables created with
<command>CREATE SEQUENCE</>. A sequence object is usually used to
generate unique identifiers for rows of a table. The sequence functions
provide simple, multi-user-safe methods for obtaining successive
sequence values from sequence objects.
</para>
<para>
For largely historical reasons, the sequence to be operated on by
a sequence-function call is specified by a text-string argument.
To achieve some compatibility with the handling of ordinary SQL
names, the sequence functions convert their argument to lower case
unless the string is double-quoted. Thus
<programlisting>
nextval('foo') <lineannotation>operates on sequence </><literal>foo</>
nextval('FOO') <lineannotation>operates on sequence </><literal>foo</>
nextval('"Foo"') <lineannotation>operates on sequence </><literal>Foo</>
</programlisting>
Of course, the text argument can be the result of an expression,
not only a simple literal, which is occasionally useful.
</para>
<para>
The available sequence functions are:
<variablelist>
<varlistentry>
<term><function>nextval</></term>
<listitem>
<para>
Advance the sequence object to its next value and return that
value. This is done atomically: even if multiple server processes
execute <function>nextval</> concurrently, each will safely receive
a distinct sequence value.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>currval</></term>
<listitem>
<para>
Return the value most recently obtained by <function>nextval</>
for this sequence in the current server process. (An error is
reported if <function>nextval</> has never been called for this
sequence in this process.) Notice that because this is returning
a process-local value, it gives a predictable answer even if other
server processes are executing <function>nextval</> meanwhile.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>setval</></term>
<listitem>
<para>
Reset the sequence object's counter value. The two-parameter
form sets the sequence's <literal>last_value</> field to the specified
value and sets its <literal>is_called</> field to <literal>true</>,
meaning that the next <function>nextval</> will advance the sequence
before returning a value. In the three-parameter form,
<literal>is_called</> may be set either <literal>true</> or
<literal>false</>. If it's set to <literal>false</>,
the next <function>nextval</> will return exactly the specified
value, and sequence advancement commences with the following
<function>nextval</>. For example,
</para>
<informalexample>
<screen>
SELECT setval('foo', 42); <lineannotation>Next nextval() will return 43</>
SELECT setval('foo', 42, true); <lineannotation>Same as above</>
SELECT setval('foo', 42, false); <lineannotation>Next nextval() will return 42</>
</screen>
</informalexample>
<para>
The result returned by <function>setval</> is just the value of its
second argument.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<important>
<para>
To avoid blocking of concurrent transactions that obtain numbers from the
same sequence, a <function>nextval</> operation is never rolled back;
that is, once a value has been fetched it is considered used, even if the
transaction that did the <function>nextval</> later aborts. This means
that aborted transactions may leave unused <quote>holes</quote> in the
sequence of assigned values. <function>setval</> operations are never
rolled back, either.
</para>
</important>
<para>
If a sequence object has been created with default parameters,
<function>nextval()</> calls on it will return successive values
beginning with one. Other behaviors can be obtained by using
special parameters in the <command>CREATE SEQUENCE</> command;
see its command reference page for more information.
</para>
</sect1>
<sect1 id="functions-conditional">
<title>Conditional Expressions</title>

96
doc/src/sgml/ref/create_sequence.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_sequence.sgml,v 1.20 2001/09/13 15:55:24 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_sequence.sgml,v 1.21 2001/11/17 22:20:34 tgl Exp $
Postgres documentation
-->
@ -15,7 +15,7 @@ Postgres documentation
CREATE SEQUENCE
</refname>
<refpurpose>
define a new sequence
define a new sequence generator
</refpurpose>
</refnamediv>
<refsynopsisdiv>
@ -42,8 +42,8 @@ CREATE [ TEMPORARY | TEMP ] SEQUENCE <replaceable class="parameter">seqname</rep
<term>TEMPORARY or TEMP</term>
<listitem>
<para>
If specified, the sequence is created only for this session, and is
automatically dropped on session exit.
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.
</para>
@ -141,6 +141,8 @@ CREATE [ TEMPORARY | TEMP ] SEQUENCE <replaceable class="parameter">seqname</rep
<replaceable class="parameter">minvalue</replaceable> or
<replaceable class="parameter">maxvalue</replaceable>,
respectively.
Without CYCLE, after the limit is reached <function>nextval</> calls
will return an error.
</para>
</listitem>
</varlistentry>
@ -222,81 +224,57 @@ ERROR: DefineSequence: MINVALUE (<replaceable class="parameter">min</replaceabl
</title>
<para>
<command>CREATE SEQUENCE</command> will enter a new sequence number generator
into the current data base. This involves creating and initializing a
into the current database. This involves creating and initializing a
new single-row
table with the name <replaceable class="parameter">seqname</replaceable>.
The generator will be owned by the user issuing the command.
</para>
<para>
After a sequence is created, you may use the function
<function>nextval('<replaceable class="parameter">seqname</replaceable>')</function>
to get a new number from the sequence.
The function
<function>currval('<replaceable class="parameter">seqname</replaceable>')</function>
may be used to determine the number returned by the last call to
<function>nextval('<replaceable class="parameter">seqname</replaceable>')</function>
for the specified sequence in the current session.
The function
<function>setval('<replaceable class="parameter">seqname</replaceable>',
<replaceable class="parameter">newvalue</replaceable>)</function>
may be used to set the current value of the specified sequence.
The next call to
<function>nextval('<replaceable class="parameter">seqname</replaceable>')</function>
will return the given value plus the sequence increment.
After a sequence is created, you use the functions
<function>nextval</function>,
<function>currval</function> and
<function>setval</function>
to operate on the sequence. These functions are documented in
the <citetitle>User's Guide</citetitle>.
</para>
<para>
Use a query like
Although you cannot update a sequence directly, you can use a query like
<programlisting>
SELECT * FROM <replaceable>seqname</replaceable>;
</programlisting>
to examine the parameters of a sequence.
As an alternative to fetching the
parameters from the original definition as above, you can use
<programlisting>
SELECT last_value FROM <replaceable>seqname</replaceable>;
</programlisting>
to obtain the last value allocated by any backend.
</para>
<para>
To avoid blocking of concurrent transactions
that obtain numbers from the same sequence, a nextval operation
is never rolled back; that is, once a value has been fetched it is
considered used, even if the transaction that did the nextval later
aborts. This means that aborted transactions may leave unused <quote>holes</quote>
in the sequence of assigned values. setval operations are never
rolled back, either.
to examine the parameters and current state of a sequence. In particular,
the <literal>last_value</> field of the sequence shows the last value
allocated by any backend process. (Of course, this value may be obsolete
by the time it's printed, if other processes are actively doing
<function>nextval</> calls.)
</para>
<caution>
<para>
Unexpected results may be obtained if a cache setting greater than one
Unexpected results may be obtained if a <replaceable class="parameter">cache</replaceable> setting greater than one
is used for a sequence object that will be used concurrently by multiple
backends. Each backend will allocate and cache successive sequence values
during one access to the sequence object and increase the sequence
object's last_value accordingly. Then, the next cache-1 uses of nextval
object's <literal>last_value</> accordingly. Then, the next <replaceable class="parameter">cache</replaceable>-1 uses of <function>nextval</>
within that backend simply return the preallocated values without touching
the shared object. So, numbers allocated but not used in the current session
will be lost. Furthermore, although multiple backends are guaranteed to
the shared object. So, any numbers allocated but not used within a session
will be lost when that session ends. Furthermore, although multiple backends are guaranteed to
allocate distinct sequence values, the values may be generated out of
sequence when all the backends are considered. (For example, with a cache
sequence when all the backends are considered. (For example, with a <replaceable class="parameter">cache</replaceable>
setting of 10, backend A might reserve values 1..10 and return nextval=1,
then
backend B might reserve values 11..20 and return nextval=11 before backend
A has generated nextval=2.) Thus, with a cache setting of one it is safe
to assume that nextval values are generated sequentially; with a cache
setting greater than one you should only assume that the nextval values
A has generated nextval=2.) Thus, with a <replaceable class="parameter">cache</replaceable> setting of one it is safe
to assume that <function>nextval</> values are generated sequentially; with a <replaceable class="parameter">cache</replaceable>
setting greater than one you should only assume that the <function>nextval</> values
are all distinct, not that they are generated purely sequentially.
Also, last_value will reflect the latest value reserved by any backend,
whether or not it has yet been returned by nextval.
Another consideration is that a setval executed on such a sequence
Also, <literal>last_value</> will reflect the latest value reserved by any backend,
whether or not it has yet been returned by <function>nextval</>.
Another consideration is that a <function>setval</> executed on such a sequence
will not be noticed by other backends until they have used up any
preallocated values they have cached.
</para>
@ -313,7 +291,8 @@ SELECT last_value FROM <replaceable>seqname</replaceable>;
Use <command>DROP SEQUENCE</command> to remove a sequence.
</para>
<para>
Each backend uses its own cache to store preallocated numbers.
When <replaceable class="parameter">cache</replaceable> is greater than
one, each backend uses its own cache to store preallocated numbers.
Numbers that are cached but not used in the current session will be
lost, resulting in <quote>holes</quote> in the sequence.
</para>
@ -333,7 +312,7 @@ CREATE SEQUENCE serial START 101;
<para>
Select the next number from this sequence:
<programlisting>
SELECT NEXTVAL ('serial');
SELECT nextval('serial');
nextval
-------
@ -343,19 +322,16 @@ nextval
<para>
Use this sequence in an INSERT:
<programlisting>
INSERT INTO distributors VALUES (NEXTVAL('serial'),'nothing');
INSERT INTO distributors VALUES (nextval('serial'), 'nothing');
</programlisting>
</para>
<para>
Set the sequence value after a COPY FROM:
Update the sequence value after a COPY FROM:
<programlisting>
CREATE FUNCTION distributors_id_max() RETURNS INT4
AS 'SELECT max(id) FROM distributors'
LANGUAGE 'sql';
BEGIN;
COPY distributors FROM 'input_file';
SELECT setval('serial', distributors_id_max());
SELECT setval('serial', max(id)) FROM distributors;
END;
</programlisting>
</para>

6
doc/src/sgml/ref/grant.sgml
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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/grant.sgml,v 1.13 2001/11/11 19:24:28 momjian Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/grant.sgml,v 1.14 2001/11/17 22:20:34 tgl Exp $
Postgres documentation
-->
@ -74,8 +74,8 @@ GRANT { { SELECT | INSERT | UPDATE | DELETE | RULE | REFERENCES | TRIGGER } [,..
specified table. <literal>SELECT ... FOR UPDATE</literal>
also requires this privilege (besides the
<literal>SELECT</literal> privilege). For sequences, this
privilege allows the use of <function>currval</function> and
<function>nextval</function>.
privilege allows the use of <function>nextval</function>,
<function>currval</function> and <function>setval</function>.
</para>
</listitem>
</varlistentry>

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