seconds will ever be shown as fractional
     on output.
    </para>

    <para>
     <xref linkend="datatype-interval-input-examples"/> shows some examples
     of valid <type>interval</type> input.
    </para>

     <table id="datatype-interval-input-examples">
      <title>Interval Input</title>
      <tgroup cols="2">
       <thead>
        <row>
         <entry>Example</entry>
         <entry>Description</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry><literal>1-2</literal></entry>
         <entry>SQL standard format: 1 year 2 months</entry>
        </row>
        <row>
         <entry><literal>3 4:05:06</literal></entry>
         <entry>SQL standard format: 3 days 4 hours 5 minutes 6 seconds</entry>
        </row>
        <row>
         <entry><literal>1 year 2 months 3 days 4 hours 5 minutes 6 seconds</literal></entry>
         <entry>Traditional Postgres format: 1 year 2 months 3 days 4 hours 5 minutes 6 seconds</entry>
        </row>
        <row>
         <entry><literal>P1Y2M3DT4H5M6S</literal></entry>
         <entry>ISO 8601 <quote>format with designators</quote>: same meaning as above</entry>
        </row>
        <row>
         <entry><literal>P0001-02-03T04:05:06</literal></entry>
         <entry>ISO 8601 <quote>alternative format</quote>: same meaning as above</entry>
        </row>
       </tbody>
      </tgroup>
     </table>

   </sect2>

   <sect2 id="datatype-interval-output">
    <title>Interval Output</title>

    <indexterm>
     <primary>interval</primary>
     <secondary>output format</secondary>
     <seealso>formatting</seealso>
    </indexterm>

    <para>
     As previously explained, <productname>PostgreSQL</productname>
     stores <type>interval</type> values as months, days, and
     microseconds.  For output, the months field is converted to years and
     months by dividing by 12.  The days field is shown as-is.  The
     microseconds field is converted to hours, minutes, seconds, and
     fractional seconds.  Thus months, minutes, and seconds will never be
     shown as exceeding the ranges 0&ndash;11, 0&ndash;59, and 0&ndash;59
     respectively, while the displayed years, days, and hours fields can
     be quite large.  (The <link
     linkend="function-justify-days"><function>justify_days</function></link>
     and <link
     linkend="function-justify-hours"><function>justify_hours</function></link>
     functions can be used if it is desirable to transpose large days or
     hours values into the next higher field.)
    </para>

    <para>
     The output format of the interval type can be set to one of the
     four styles <literal>sql_standard</literal>, <literal>postgres</literal>,
     <literal>postgres_verbose</literal>, or <literal>iso_8601</literal>,
     using the command <literal>SET intervalstyle</literal>.
     The default is the <literal>postgres</literal> format.
     <xref linkend="interval-style-output-table"/> shows examples of each
     output style.
    </para>

    <para>
     The <literal>sql_standard</literal> style produces output that conforms to
     the SQL standard's specification for interval literal strings, if
     the interval value meets the standard's restrictions (either year-month
     only or day-time only, with no mixing of positive
     and negative components).  Otherwise the output looks like a standard
     year-month literal string followed by a day-time literal string,
     with explicit signs added to disambiguate mixed-sign intervals.
    </para>

    <para>
     The output of the <literal>postgres</literal> style matches the output of
     <productname>PostgreSQL</productname> releases prior to 8.4 when the
     <xref linkend="guc-datestyle"/> parameter was set to <literal>ISO</literal>.
    </para>

    <para>
     The output of the <literal>postgres_verbose</literal> style matches the output of
     <productname>PostgreSQL</productname> releases prior to 8.4 when the
     <varname>DateStyle</varname> parameter