383 lines
12 KiB
Plaintext
383 lines
12 KiB
Plaintext
<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_sequence.sgml,v 1.14 2000/10/05 19:48:17 momjian Exp $
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Postgres documentation
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-->
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<refentry id="SQL-CREATESEQUENCE">
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<refmeta>
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<refentrytitle id="sql-createsequence-title">
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CREATE SEQUENCE
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</refentrytitle>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>
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CREATE SEQUENCE
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</refname>
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<refpurpose>
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Creates a new sequence number generator
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</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<refsynopsisdivinfo>
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<date>1999-07-20</date>
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</refsynopsisdivinfo>
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<synopsis>
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CREATE SEQUENCE <replaceable class="parameter">seqname</replaceable> [ INCREMENT <replaceable class="parameter">increment</replaceable> ]
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[ MINVALUE <replaceable class="parameter">minvalue</replaceable> ] [ MAXVALUE <replaceable class="parameter">maxvalue</replaceable> ]
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[ START <replaceable class="parameter">start</replaceable> ] [ CACHE <replaceable class="parameter">cache</replaceable> ] [ CYCLE ]
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</synopsis>
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<refsect2 id="R2-SQL-CREATESEQUENCE-1">
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<refsect2info>
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<date>1998-09-11</date>
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</refsect2info>
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<title>
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Inputs
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</title>
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<para>
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<variablelist>
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<varlistentry>
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<term><replaceable class="parameter">seqname</replaceable></term>
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<listitem>
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<para>
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The name of a sequence to be created.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">increment</replaceable></term>
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<listitem>
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<para>
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The
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<option>INCREMENT <replaceable class="parameter">increment</replaceable></option>
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clause is optional. A positive value will make an
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ascending sequence, a negative one a descending sequence.
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The default value is one (1).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">minvalue</replaceable></term>
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<listitem>
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<para>
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The optional clause <option>MINVALUE
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<replaceable class="parameter">minvalue</replaceable></option>
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determines the minimum value
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a sequence can generate. The defaults are 1 and -2147483647 for
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ascending and descending sequences, respectively.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">maxvalue</replaceable></term>
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<listitem>
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<para>
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Use the optional clause <option>MAXVALUE
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<replaceable class="parameter">maxvalue</replaceable></option> to
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determine the maximum
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value for the sequence. The defaults are 2147483647 and -1 for
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ascending and descending sequences, respectively.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">start</replaceable></term>
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<listitem>
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<para>
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The optional <option>START
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<replaceable class="parameter">start</replaceable>
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clause</option> enables the sequence to begin anywhere.
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The default starting value is
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<replaceable class="parameter">minvalue</replaceable>
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for ascending sequences and
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<replaceable class="parameter">maxvalue</replaceable>
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for descending ones.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">cache</replaceable></term>
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<listitem>
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<para>
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The <option>CACHE <replaceable class="parameter">cache</replaceable></option> option
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enables sequence numbers to be preallocated
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and stored in memory for faster access. The minimum
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value is 1 (only one value can be generated at a time, i.e., no cache)
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and this is also the default.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>CYCLE</term>
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<listitem>
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<para>
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The optional CYCLE keyword may be used to enable the sequence
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to continue when the
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<replaceable class="parameter">maxvalue</replaceable> or
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<replaceable class="parameter">minvalue</replaceable> has been
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reached by
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an ascending or descending sequence respectively. If the limit is
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reached, the next number generated will be whatever the
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<replaceable class="parameter">minvalue</replaceable> or
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<replaceable class="parameter">maxvalue</replaceable> is,
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as appropriate.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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<refsect2 id="R2-SQL-CREATESEQUENCE-2">
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<refsect2info>
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<date>1998-09-11</date>
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</refsect2info>
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<title>
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Outputs
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</title>
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<para>
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<variablelist>
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<varlistentry>
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<term><computeroutput>
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CREATE
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</computeroutput></term>
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<listitem>
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<para>
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Message returned if the command is successful.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><computeroutput>
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ERROR: Relation '<replaceable class="parameter">seqname</replaceable>' already exists
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</computeroutput></term>
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<listitem>
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<para>
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If the sequence specified already exists.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><computeroutput>
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ERROR: DefineSequence: MINVALUE (<replaceable class="parameter">start</replaceable>) can't be >= MAXVALUE (<replaceable class="parameter">max</replaceable>)
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</computeroutput></term>
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<listitem>
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<para>
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If the specified starting value is out of range.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><computeroutput>
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ERROR: DefineSequence: START value (<replaceable class="parameter">start</replaceable>) can't be < MINVALUE (<replaceable class="parameter">min</replaceable>)
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</computeroutput></term>
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<listitem>
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<para>
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If the specified starting value is out of range.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><computeroutput>
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ERROR: DefineSequence: MINVALUE (<replaceable class="parameter">min</replaceable>) can't be >= MAXVALUE (<replaceable class="parameter">max</replaceable>)
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</computeroutput></term>
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<listitem>
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<para>
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If the minimum and maximum values are inconsistant.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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</refsynopsisdiv>
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<refsect1 id="R1-SQL-CREATESEQUENCE-1">
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<refsect1info>
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<date>1998-09-11</date>
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</refsect1info>
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<title>
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Description
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</title>
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<para>
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<command>CREATE SEQUENCE</command> will enter a new sequence number generator
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into the current data base. This involves creating and initializing a
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new single-row
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table with the name <replaceable class="parameter">seqname</replaceable>.
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The generator will be "owned" by the user issuing the command.
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</para>
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<para>
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After a sequence is created, you may use the function
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<function>nextval(<replaceable class="parameter">seqname</replaceable>)</function>
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to get a new number from the sequence.
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The function
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<function>currval('<replaceable class="parameter">seqname</replaceable>')</function>
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may be used to determine the number returned by the last call to
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<function>nextval(<replaceable class="parameter">seqname</replaceable>)</function>
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for the specified sequence in the current session.
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The function
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<function>setval('<replaceable class="parameter">seqname</replaceable>',
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<replaceable class="parameter">newvalue</replaceable>)</function>
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may be used to set the current value of the specified sequence.
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The next call to
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<function>nextval(<replaceable class="parameter">seqname</replaceable>)</function>
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will return the given value plus the sequence increment.
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</para>
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<para>
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Use a query like
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<programlisting>
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SELECT * FROM <replaceable>seqname</replaceable>;
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</programlisting>
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to get the parameters of a sequence.
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As an alternative to fetching the
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parameters from the original definition as above, you can use
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<programlisting>
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SELECT last_value FROM <replaceable>seqname</replaceable>;
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</programlisting>
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to obtain the last value allocated by any backend.
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</para>
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<para>
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Low-level locking is used to enable multiple simultaneous
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calls to a generator.
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</para>
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<caution>
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<para>
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Unexpected results may be obtained if a cache setting greater than one
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is used for a sequence object that will be used concurrently by multiple
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backends. Each backend will allocate and cache successive sequence values
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during one access to the sequence object and increase the sequence
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object's last_value accordingly. Then, the next cache-1 uses of nextval
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within that backend simply return the preallocated values without touching
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the shared object. So, numbers allocated but not used in the current session
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will be lost. Furthermore, although multiple backends are guaranteed to
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allocate distinct sequence values, the values may be generated out of
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sequence when all the backends are considered. (For example, with a cache
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setting of 10, backend A might reserve values 1..10 and return nextval=1,
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then
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backend B might reserve values 11..20 and return nextval=11 before backend
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A has generated nextval=2.) Thus, with a cache setting of one it is safe
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to assume that nextval values are generated sequentially; with a cache
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setting greater than one you should only assume that the nextval values
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are all distinct, not that they are generated purely sequentially.
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Also, last_value will reflect the latest value reserved by any backend,
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whether or not it has yet been returned by nextval.
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</para>
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</caution>
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<refsect2 id="R2-SQL-CREATESEQUENCE-3">
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<refsect2info>
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<date>1998-09-11</date>
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</refsect2info>
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<title>
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Notes
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</title>
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<para>
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Use <command>DROP SEQUENCE</command> to remove a sequence.
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</para>
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<para>
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Each backend uses its own cache to store allocated numbers.
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Numbers that are cached but not used in the current session will be
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lost, resulting in "holes" in the sequence.
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</para>
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</refsect2>
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</refsect1>
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<refsect1 id="R1-SQL-CREATESEQUENCE-2">
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<title>
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Usage
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</title>
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<para>
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Create an ascending sequence called <literal>serial</literal>, starting at 101:
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</para>
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<programlisting>
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CREATE SEQUENCE serial START 101;
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</programlisting>
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<para>
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Select the next number from this sequence:
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<programlisting>
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SELECT NEXTVAL ('serial');
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nextval
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-------
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114
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</programlisting>
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</para>
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<para>
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Use this sequence in an INSERT:
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<programlisting>
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INSERT INTO distributors VALUES (NEXTVAL('serial'),'nothing');
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</programlisting>
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</para>
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<para>
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Set the sequence value after a COPY FROM:
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<programlisting>
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CREATE FUNCTION distributors_id_max() RETURNS INT4
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AS 'SELECT max(id) FROM distributors'
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LANGUAGE 'sql';
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BEGIN;
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COPY distributors FROM 'input_file';
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SELECT setval('serial', distributors_id_max());
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END;
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</programlisting>
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</para>
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</refsect1>
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<refsect1 id="R1-SQL-CREATESEQUENCE-3">
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<title>
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Compatibility
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</title>
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<refsect2 id="R2-SQL-CREATESEQUENCE-4">
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<refsect2info>
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<date>1998-09-11</date>
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</refsect2info>
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<title>
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SQL92
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</title>
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<para>
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<command>CREATE SEQUENCE</command> is a <productname>Postgres</productname>
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language extension.
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There is no <command>CREATE SEQUENCE</command> statement
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in <acronym>SQL92</acronym>.
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</para>
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</refsect2>
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</refsect1>
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</refentry>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:nil
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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sgml-parent-document:nil
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sgml-default-dtd-file:"../reference.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:"/usr/lib/sgml/catalog"
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sgml-local-ecat-files:nil
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End:
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-->
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