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>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,