data_type  *key = DatumGetDataType(entry->key);
    bool        retval;

    /*
     * determine return value as a function of strategy, key and query.
     *
     * Use GIST_LEAF(entry) to know where you're called in the index tree,
     * which comes handy when supporting the = operator for example (you could
     * check for non empty union() in non-leaf nodes and equality in leaf
     * nodes).
     */

    *recheck = true;        /* or false if check is exact */

    PG_RETURN_BOOL(retval);
}
</programlisting>

       Here, <varname>key</varname> is an element in the index and <varname>query</varname>
       the value being looked up in the index. The <literal>StrategyNumber</literal>
       parameter indicates which operator of your operator class is being
       applied &mdash; it matches one of the operator numbers in the
       <command>CREATE OPERATOR CLASS</command> command.
      </para>

      <para>
       Depending on which operators you have included in the class, the data
       type of <varname>query</varname> could vary with the operator, since it will
       be whatever type is on the right-hand side of the operator, which might
       be different from the indexed data type appearing on the left-hand side.
       (The above code skeleton assumes that only one type is possible; if
       not, fetching the <varname>query</varname> argument value would have to depend
       on the operator.)  It is recommended that the SQL declaration of
       the <function>consistent</function> function use the opclass's indexed data
       type for the <varname>query</varname> argument, even though the actual type
       might be something else depending on the operator.
      </para>

     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>union</function></term>
     <listitem>
      <para>
       This method consolidates information in the tree.  Given a set of
       entries, this function generates a new index entry that represents
       all the given entries.
      </para>

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

<programlisting>
CREATE OR REPLACE FUNCTION my_union(internal, internal)
RETURNS storage_type
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_union);

Datum
my_union(PG_FUNCTION_ARGS)
{
    GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
    GISTENTRY  *ent = entryvec-&gt;vector;
    data_type  *out,
               *tmp,
               *old;
    int         numranges,
                i = 0;

    numranges = entryvec-&gt;n;
    tmp = DatumGetDataType(ent[0].key);
    out = tmp;

    if (numranges == 1)
    {
        out = data_type_deep_copy(tmp);

        PG_RETURN_DATA_TYPE_P(out);
    }

    for (i = 1; i &lt; numranges; i++)
    {
        old = out;
        tmp = DatumGetDataType(ent[i].key);
        out = my_union_implementation(out, tmp);
    }

    PG_RETURN_DATA_TYPE_P(out);
}
</programlisting>
      </para>

      <para>
        As you can see, in this skeleton we're dealing with a data type
        where <literal>union(X, Y, Z) = union(union(X, Y), Z)</literal>. It's easy
        enough to support data types where this is not the case, by
        implementing the proper union algorithm in this
        <acronym>GiST</acronym> support method.
      </para>

      <para>
        The result of the <function>union</function> function must be a value of the
        index's storage type, whatever that is (it might or might not be
        different from the indexed column's type).  The <function>union</function>
        function should return a pointer to newly <function>palloc()</function>ed
        memory. You can't just return the input value as-is, even if there is
        no type change.
      </para>

      <para>
       As shown above, the <function>union</function>