unionR
         * accordingly. Append the entries to either v->spl_left or
         * v->spl_right, and care about the counters.
         */

        if (my_choice_is_left(unionL, curl, unionR, curr))
        {
            if (unionL == NULL)
                unionL = tmp_union;
            else
                unionL = my_union_implementation(unionL, tmp_union);

            *left = real_index;
            ++left;
            ++(v->spl_nleft);
        }
        else
        {
            /*
             * Same on the right
             */
        }
    }

    v->spl_ldatum = DataTypeGetDatum(unionL);
    v->spl_rdatum = DataTypeGetDatum(unionR);
    PG_RETURN_POINTER(v);
}
</programlisting>

       Notice that the <function>picksplit</function> function's result is delivered
       by modifying the passed-in <structname>v</structname> structure.  The return
       value per se is ignored, though it's conventional to pass back the
       address of <structname>v</structname>.
      </para>

      <para>
        Like <function>penalty</function>, the <function>picksplit</function> function
        is crucial to good performance of the index.  Designing suitable
        <function>penalty</function> and <function>picksplit</function> implementations
        is where the challenge of implementing well-performing
        <acronym>GiST</acronym> indexes lies.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>same</function></term>
     <listitem>
      <para>
       Returns true if two index entries are identical, false otherwise.
       (An <quote>index entry</quote> is a value of the index's storage type,
       not necessarily the original indexed column's type.)
      </para>

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

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

        And the matching code in the C module could then follow this skeleton:

<programlisting>
PG_FUNCTION_INFO_V1(my_same);

Datum
my_same(PG_FUNCTION_ARGS)
{
    prefix_range *v1 = PG_GETARG_PREFIX_RANGE_P(0);
    prefix_range *v2 = PG_GETARG_PREFIX_RANGE_P(1);
    bool       *result = (bool *) PG_GETARG_POINTER(2);

    *result = my_eq(v1, v2);
    PG_RETURN_POINTER(result);
}
</programlisting>

        For historical reasons, the <function>same</function> function doesn't
        just return a Boolean result; instead it has to store the flag
        at the location indicated by the third argument.  The return
        value per se is ignored, though it's conventional to pass back the
        address of that argument.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>distance</function></term>
     <listitem>
      <para>
       Given an index entry <literal>p</literal> and a query value <literal>q</literal>,
       this function determines the index entry's
       <quote>distance</quote> from the query value.  This function must be
       supplied if the operator class contains any ordering operators.
       A query using the ordering operator will be implemented by returning
       index entries with the smallest <quote>distance</quote> values first,
       so the results must be consistent with the operator's semantics.
       For a leaf index entry the result just represents the distance to
       the index entry; for an internal tree node, the result must be the
       smallest distance that any child entry could have.
      </para>

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

<programlisting>
CREATE OR REPLACE FUNCTION my_distance(internal, data_type, smallint, oid, internal)
RETURNS float8
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>

        And the matching code in the C module could then follow this skeleton:

<programlisting>