distance function's result values can be any finite <type>float8</type>
       values, so long as the relative order of the result values matches the
       order returned by the ordering operator.  (Infinity and minus infinity
       are used internally to handle cases such as nulls, so it is not
       recommended that <function>distance</function> functions return these values.)
      </para>

     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>fetch</function></term>
     <listitem>
      <para>
       Converts the compressed index representation of a data item into the
       original data type, for index-only scans. The returned data must be an
       exact, non-lossy copy of the originally indexed value.
      </para>

      <para>
        The <acronym>SQL</acronym> declaration of the function must look like this:

<programlisting>
CREATE OR REPLACE FUNCTION my_fetch(internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>

        The argument is a pointer to a <structname>GISTENTRY</structname> struct. On
        entry, its <structfield>key</structfield> field contains a non-NULL leaf datum in
        compressed form. The return value is another <structname>GISTENTRY</structname>
        struct, whose <structfield>key</structfield> field contains the same datum in its
        original, uncompressed form. If the opclass's compress function does
        nothing for leaf entries, the <function>fetch</function> method can return the
        argument as-is.  Or, if the opclass does not have a compress function,
        the <function>fetch</function> method can be omitted as well, since it would
        necessarily be a no-op.
       </para>

       <para>
        The matching code in the C module could then follow this skeleton:

<programlisting>
PG_FUNCTION_INFO_V1(my_fetch);

Datum
my_fetch(PG_FUNCTION_ARGS)
{
    GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
    input_data_type *in = DatumGetPointer(entry->key);
    fetched_data_type *fetched_data;
    GISTENTRY  *retval;

    retval = palloc(sizeof(GISTENTRY));
    fetched_data = palloc(sizeof(fetched_data_type));

    /*
     * Convert 'fetched_data' into the a Datum of the original datatype.
     */

    /* fill *retval from fetched_data. */
    gistentryinit(*retval, PointerGetDatum(converted_datum),
                  entry->rel, entry->page, entry->offset, FALSE);

    PG_RETURN_POINTER(retval);
}
</programlisting>
      </para>

      <para>
       If the compress method is lossy for leaf entries, the operator class
       cannot support index-only scans, and must not define
       a <function>fetch</function> function.
      </para>

     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>options</function></term>
     <listitem>
      <para>
       Allows definition of user-visible parameters that control operator
       class behavior.
      </para>

      <para>
        The <acronym>SQL</acronym> declaration of the function must look like this:

<programlisting>
CREATE OR REPLACE FUNCTION my_options(internal)
RETURNS void
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
      </para>

      <para>
       The function is passed a pointer to a <structname>local_relopts</structname>
       struct, which needs to be filled with a set of operator class
       specific options.  The options can be accessed from other support
       functions using the <literal>PG_HAS_OPCLASS_OPTIONS()</literal> and
       <literal>PG_GET_OPCLASS_OPTIONS()</literal> macros.
      </para>

       <para>
        An example implementation of my_options() and parameters use
        from other support functions are given below:

<programlisting>
typedef enum MyEnumType
{
    MY_ENUM_ON,
    MY_ENUM_OFF,
    MY_ENUM_AUTO
} MyEnumType;

typedef struct
{
    int32   vl_len_;    /* varlena header (do not touch directly!) */
    int     int_param;  /* integer parameter */
    double  real_param; /*