<replaceable>B</replaceable>,
     respectively.  A null result is disallowed: all values of the
     data type must be comparable.  See
     <filename>src/backend/access/nbtree/nbtcompare.c</filename> for
     examples.
    </para>

    <para>
     If the compared values are of a collatable data type, the
     appropriate collation OID will be passed to the comparison
     support function, using the standard
     <function>PG_GET_COLLATION()</function> mechanism.
    </para>
   </listitem>
  </varlistentry>
  <varlistentry>
   <term><function>sortsupport</function></term>
   <listitem>
    <para>
     Optionally, a btree operator family may provide <firstterm>sort
      support</firstterm> function(s), registered under support
     function number 2.  These functions allow implementing
     comparisons for sorting purposes in a more efficient way than
     naively calling the comparison support function.  The APIs
     involved in this are defined in
     <filename>src/include/utils/sortsupport.h</filename>.
    </para>
   </listitem>
  </varlistentry>
  <varlistentry>
   <term><function>in_range</function></term>
   <listitem>
    <indexterm>
     <primary>in_range support functions</primary>
    </indexterm>

    <indexterm>
     <primary>support functions</primary>
     <secondary>in_range</secondary>
    </indexterm>
    <para>
     Optionally, a btree operator family may provide
     <firstterm>in_range</firstterm> support function(s), registered
     under support function number 3.  These are not used during btree
     index operations; rather, they extend the semantics of the
     operator family so that it can support window clauses containing
     the <literal>RANGE</literal> <replaceable>offset</replaceable>
     <literal>PRECEDING</literal> and <literal>RANGE</literal>
     <replaceable>offset</replaceable> <literal>FOLLOWING</literal>
     frame bound types (see <xref
      linkend="syntax-window-functions"/>).  Fundamentally, the extra
     information provided is how to add or subtract an
     <replaceable>offset</replaceable> value in a way that is
     compatible with the family's data ordering.
    </para>

    <para>
     An <function>in_range</function> function must have the signature
<synopsis>
in_range(<replaceable>val</replaceable> type1, <replaceable>base</replaceable> type1, <replaceable>offset</replaceable> type2, <replaceable>sub</replaceable> bool, <replaceable>less</replaceable> bool)
returns bool
</synopsis>
     <replaceable>val</replaceable> and
     <replaceable>base</replaceable> must be of the same type, which
     is one of the types supported by the operator family (i.e., a
     type for which it provides an ordering).  However,
     <replaceable>offset</replaceable> could be of a different type,
     which might be one otherwise unsupported by the family.  An
     example is that the built-in <literal>time_ops</literal> family
     provides an <function>in_range</function> function that has
     <replaceable>offset</replaceable> of type <type>interval</type>.
     A family can provide <function>in_range</function> functions for
     any of its supported types and one or more
     <replaceable>offset</replaceable> types.  Each
     <function>in_range</function> function should be entered in
     <structname>pg_amproc</structname> with
     <structfield>amproclefttype</structfield> equal to
     <type>type1</type> and <structfield>amprocrighttype</structfield>
     equal to <type>type2</type>.
    </para>

    <para>
     The essential semantics of an <function>in_range</function>
     function depend on the two Boolean flag parameters.  It should
     add or subtract <replaceable>base</replaceable> and
     <replaceable>offset</replaceable>, then compare
     <replaceable>val</replaceable> to the result, as follows:
     <itemizedlist>
      <listitem>
       <para>
        if <literal>!</literal><replaceable>sub</replaceable> and
        <literal>!</literal><replaceable>less</replaceable>,