non-<literal>NaN</literal> values.
     </para>
    </note>

    <para>
     <productname>PostgreSQL</productname> also supports the SQL-standard
     notations <type>float</type> and
     <type>float(<replaceable>p</replaceable>)</type> for specifying
     inexact numeric types.  Here, <replaceable>p</replaceable> specifies
     the minimum acceptable precision in <emphasis>binary</emphasis> digits.
     <productname>PostgreSQL</productname> accepts
     <type>float(1)</type> to <type>float(24)</type> as selecting the
     <type>real</type> type, while
     <type>float(25)</type> to <type>float(53)</type> select
     <type>double precision</type>.  Values of <replaceable>p</replaceable>
     outside the allowed range draw an error.
     <type>float</type> with no precision specified is taken to mean
     <type>double precision</type>.
    </para>

   </sect2>

   <sect2 id="datatype-serial">
    <title>Serial Types</title>

    <indexterm zone="datatype-serial">
     <primary>smallserial</primary>
    </indexterm>

    <indexterm zone="datatype-serial">
     <primary>serial</primary>
    </indexterm>

    <indexterm zone="datatype-serial">
     <primary>bigserial</primary>
    </indexterm>

    <indexterm zone="datatype-serial">
     <primary>serial2</primary>
    </indexterm>

    <indexterm zone="datatype-serial">
     <primary>serial4</primary>
    </indexterm>

    <indexterm zone="datatype-serial">
     <primary>serial8</primary>
    </indexterm>

    <indexterm>
     <primary>auto-increment</primary>
     <see>serial</see>
    </indexterm>

    <indexterm>
     <primary>sequence</primary>
     <secondary>and serial type</secondary>
    </indexterm>

    <note>
     <para>
      This section describes a PostgreSQL-specific way to create an
      autoincrementing column.  Another way is to use the SQL-standard
      identity column feature, described at <xref linkend="ddl-identity-columns"/>.
     </para>
    </note>

    <para>
     The data types <type>smallserial</type>, <type>serial</type> and
     <type>bigserial</type> are not true types, but merely
     a notational convenience for creating unique identifier columns
     (similar to the <literal>AUTO_INCREMENT</literal> property
     supported by some other databases). In the current
     implementation, specifying:

<programlisting>
CREATE TABLE <replaceable class="parameter">tablename</replaceable> (
    <replaceable class="parameter">colname</replaceable> SERIAL
);
</programlisting>

     is equivalent to specifying:

<programlisting>
CREATE SEQUENCE <replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq AS integer;
CREATE TABLE <replaceable class="parameter">tablename</replaceable> (
    <replaceable class="parameter">colname</replaceable> integer NOT NULL DEFAULT nextval('<replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq')
);
ALTER SEQUENCE <replaceable class="parameter">tablename</replaceable>_<replaceable class="parameter">colname</replaceable>_seq OWNED BY <replaceable class="parameter">tablename</replaceable>.<replaceable class="parameter">colname</replaceable>;
</programlisting>

     Thus, we have created an integer column and arranged for its default
     values to be assigned from a sequence generator.  A <literal>NOT NULL</literal>
     constraint is applied to ensure that a null value cannot be
     inserted.  (In most cases you would also want to attach a
     <literal>UNIQUE</literal> or <literal>PRIMARY KEY</literal> constraint to prevent
     duplicate values from being inserted by accident, but this is
     not automatic.)  Lastly, the sequence is marked as <quote>owned by</quote>
     the column, so that it will be dropped if the column or table is dropped.
    </para>

    <note>
      <para>
        Because <type>smallserial</type>, <type>serial</type> and
        <type>bigserial</type> are implemented using sequences, there