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diff --git a/doc/src/sgml/ref/create_sequence.sgml b/doc/src/sgml/ref/create_sequence.sgml new file mode 100644 index 0000000..34e9084 --- /dev/null +++ b/doc/src/sgml/ref/create_sequence.sgml @@ -0,0 +1,413 @@ +<!-- +doc/src/sgml/ref/create_sequence.sgml +PostgreSQL documentation +--> + +<refentry id="sql-createsequence"> + <indexterm zone="sql-createsequence"> + <primary>CREATE SEQUENCE</primary> + </indexterm> + + <refmeta> + <refentrytitle>CREATE SEQUENCE</refentrytitle> + <manvolnum>7</manvolnum> + <refmiscinfo>SQL - Language Statements</refmiscinfo> + </refmeta> + + <refnamediv> + <refname>CREATE SEQUENCE</refname> + <refpurpose>define a new sequence generator</refpurpose> + </refnamediv> + + <refsynopsisdiv> +<synopsis> +CREATE [ { TEMPORARY | TEMP } | UNLOGGED ] SEQUENCE [ IF NOT EXISTS ] <replaceable class="parameter">name</replaceable> + [ AS <replaceable class="parameter">data_type</replaceable> ] + [ INCREMENT [ BY ] <replaceable class="parameter">increment</replaceable> ] + [ MINVALUE <replaceable class="parameter">minvalue</replaceable> | NO MINVALUE ] [ MAXVALUE <replaceable class="parameter">maxvalue</replaceable> | NO MAXVALUE ] + [ START [ WITH ] <replaceable class="parameter">start</replaceable> ] [ CACHE <replaceable class="parameter">cache</replaceable> ] [ [ NO ] CYCLE ] + [ OWNED BY { <replaceable class="parameter">table_name</replaceable>.<replaceable class="parameter">column_name</replaceable> | NONE } ] +</synopsis> + </refsynopsisdiv> + + <refsect1> + <title>Description</title> + + <para> + <command>CREATE SEQUENCE</command> creates a new sequence number + generator. This involves creating and initializing a new special + single-row table with the name <replaceable + class="parameter">name</replaceable>. The generator will be + owned by the user issuing the command. + </para> + + <para> + 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. + </para> + + <para> + 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 + <xref linkend="functions-sequence"/>. + </para> + + <para> + Although you cannot update a sequence directly, you can use a query like: + +<programlisting> +SELECT * FROM <replaceable>name</replaceable>; +</programlisting> + + to examine the parameters and current state of a sequence. In particular, + the <literal>last_value</literal> 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 + <function>nextval</function> calls.) + </para> + </refsect1> + + <refsect1> + <title>Parameters</title> + + <variablelist> + <varlistentry> + <term><literal>TEMPORARY</literal> or <literal>TEMP</literal></term> + <listitem> + <para> + 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. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>UNLOGGED</literal></term> + <listitem> + <para> + 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. + </para> + + <para> + 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. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>IF NOT EXISTS</literal></term> + <listitem> + <para> + 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. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">name</replaceable></term> + <listitem> + <para> + The name (optionally schema-qualified) of the sequence to be created. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">data_type</replaceable></term> + <listitem> + <para> + The optional + clause <literal>AS <replaceable class="parameter">data_type</replaceable></literal> + specifies the data type of the sequence. Valid types are + <literal>smallint</literal>, <literal>integer</literal>, + and <literal>bigint</literal>. <literal>bigint</literal> is the + default. The data type determines the default minimum and maximum + values of the sequence. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">increment</replaceable></term> + <listitem> + <para> + The optional clause <literal>INCREMENT BY <replaceable + class="parameter">increment</replaceable></literal> 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. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">minvalue</replaceable></term> + <term><literal>NO MINVALUE</literal></term> + <listitem> + <para> + The optional clause <literal>MINVALUE <replaceable + class="parameter">minvalue</replaceable></literal> determines + the minimum value a sequence can generate. If this clause is not + supplied or <option>NO MINVALUE</option> 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. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">maxvalue</replaceable></term> + <term><literal>NO MAXVALUE</literal></term> + <listitem> + <para> + The optional clause <literal>MAXVALUE <replaceable + class="parameter">maxvalue</replaceable></literal> determines + the maximum value for the sequence. If this clause is not + supplied or <option>NO MAXVALUE</option> 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. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">start</replaceable></term> + <listitem> + <para> + The optional clause <literal>START WITH <replaceable + class="parameter">start</replaceable> </literal> allows the + sequence to begin anywhere. The default starting value is + <replaceable class="parameter">minvalue</replaceable> for + ascending sequences and <replaceable + class="parameter">maxvalue</replaceable> for descending ones. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><replaceable class="parameter">cache</replaceable></term> + <listitem> + <para> + The optional clause <literal>CACHE <replaceable + class="parameter">cache</replaceable></literal> 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. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>CYCLE</literal></term> + <term><literal>NO CYCLE</literal></term> + <listitem> + <para> + The <literal>CYCLE</literal> option allows the sequence to wrap + around when the <replaceable + class="parameter">maxvalue</replaceable> or <replaceable + class="parameter">minvalue</replaceable> has been reached by an + ascending or descending sequence respectively. If the limit is + reached, the next number generated will be the <replaceable + class="parameter">minvalue</replaceable> or <replaceable + class="parameter">maxvalue</replaceable>, respectively. + </para> + + <para> + If <literal>NO CYCLE</literal> is specified, any calls to + <function>nextval</function> after the sequence has reached its + maximum value will return an error. If neither + <literal>CYCLE</literal> or <literal>NO CYCLE</literal> are + specified, <literal>NO CYCLE</literal> is the default. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><literal>OWNED BY</literal> <replaceable class="parameter">table_name</replaceable>.<replaceable class="parameter">column_name</replaceable></term> + <term><literal>OWNED BY NONE</literal></term> + <listitem> + <para> + The <literal>OWNED BY</literal> 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. + <literal>OWNED BY NONE</literal>, the default, specifies that there + is no such association. + </para> + </listitem> + </varlistentry> + </variablelist> + </refsect1> + + <refsect1> + <title>Notes</title> + + <para> + Use <command>DROP SEQUENCE</command> to remove a sequence. + </para> + + <para> + Sequences are based on <type>bigint</type> arithmetic, so the range + cannot exceed the range of an eight-byte integer + (-9223372036854775808 to 9223372036854775807). + </para> + + <para> + Because <function>nextval</function> and <function>setval</function> calls are never + rolled back, sequence objects cannot be used if <quote>gapless</quote> + 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. + </para> + + <para> + Unexpected results might 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 sessions. Each session will allocate and cache successive + sequence values during one access to the sequence object and + increase the sequence object's <literal>last_value</literal> accordingly. + Then, the next <replaceable class="parameter">cache</replaceable>-1 + uses of <function>nextval</function> 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 <quote>holes</quote> in the + sequence. + </para> + + <para> + 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 <replaceable class="parameter">cache</replaceable> setting of 10, + session A might reserve values 1..10 and return + <function>nextval</function>=1, then session B might reserve values + 11..20 and return <function>nextval</function>=11 before session A + has generated <function>nextval</function>=2. Thus, with a + <replaceable class="parameter">cache</replaceable> setting of one + it is safe to assume that <function>nextval</function> values are generated + sequentially; with a <replaceable + class="parameter">cache</replaceable> setting greater than one you + should only assume that the <function>nextval</function> values are all + distinct, not that they are generated purely sequentially. Also, + <literal>last_value</literal> will reflect the latest value reserved by + any session, whether or not it has yet been returned by + <function>nextval</function>. + </para> + + <para> + Another consideration is that a <function>setval</function> executed on + such a sequence will not be noticed by other sessions until they + have used up any preallocated values they have cached. + </para> + </refsect1> + + <refsect1> + <title>Examples</title> + + <para> + Create an ascending sequence called <literal>serial</literal>, starting at 101: +<programlisting> +CREATE SEQUENCE serial START 101; +</programlisting> + </para> + + <para> + Select the next number from this sequence: +<programlisting> +SELECT nextval('serial'); + + nextval +--------- + 101 +</programlisting> + </para> + + <para> + Select the next number from this sequence: +<programlisting> +SELECT nextval('serial'); + + nextval +--------- + 102 +</programlisting> + </para> + + <para> + Use this sequence in an <command>INSERT</command> command: +<programlisting> +INSERT INTO distributors VALUES (nextval('serial'), 'nothing'); +</programlisting> + </para> + + <para> + Update the sequence value after a <command>COPY FROM</command>: +<programlisting> +BEGIN; +COPY distributors FROM 'input_file'; +SELECT setval('serial', max(id)) FROM distributors; +END; +</programlisting></para> + </refsect1> + + <refsect1> + <title>Compatibility</title> + + <para> + <command>CREATE SEQUENCE</command> conforms to the <acronym>SQL</acronym> + standard, with the following exceptions: + <itemizedlist> + <listitem> + <para> + Obtaining the next value is done using the <function>nextval()</function> + function instead of the standard's <command>NEXT VALUE FOR</command> + expression. + </para> + </listitem> + <listitem> + <para> + The <literal>OWNED BY</literal> clause is a <productname>PostgreSQL</productname> + extension. + </para> + </listitem> + </itemizedlist></para> + </refsect1> + + <refsect1> + <title>See Also</title> + + <simplelist type="inline"> + <member><xref linkend="sql-altersequence"/></member> + <member><xref linkend="sql-dropsequence"/></member> + </simplelist> + </refsect1> + +</refentry> |