defined.  For prefix operators only
   <literal>RIGHTARG</literal> should be defined.
   The <replaceable class="parameter">function_name</replaceable>
   function must have been previously defined using <command>CREATE
   FUNCTION</command> and must be defined to accept the correct number
   of arguments (either one or two) of the indicated types.
  </para>

  <para>
   In the syntax of <literal>CREATE OPERATOR</literal>, the keywords
   <literal>FUNCTION</literal> and <literal>PROCEDURE</literal> are
   equivalent, but the referenced function must in any case be a function, not
   a procedure.  The use of the keyword <literal>PROCEDURE</literal> here is
   historical and deprecated.
  </para>

  <para>
   The other clauses specify optional operator optimization attributes.
   Their meaning is detailed in <xref linkend="xoper-optimization"/>.
  </para>

  <para>
   To be able to create an operator, you must have <literal>USAGE</literal>
   privilege on the argument types and the return type, as well
   as <literal>EXECUTE</literal> privilege on the underlying function.  If a
   commutator or negator operator is specified, you must own those operators.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

    <variablelist>
     <varlistentry>
      <term><replaceable class="parameter">name</replaceable></term>
      <listitem>
       <para>
        The name of the operator to be defined. See above for allowable
        characters.  The name can be schema-qualified, for example
        <literal>CREATE OPERATOR myschema.+ (...)</literal>.  If not, then
        the operator is created in the current schema.  Two operators
        in the same schema can have the same name if they operate on
        different data types.  This is called
        <firstterm>overloading</firstterm>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">function_name</replaceable></term>
      <listitem>
       <para>
        The function used to implement this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">left_type</replaceable></term>
      <listitem>
       <para>
        The data type of the operator's left operand, if any.
        This option would be omitted for a prefix operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">right_type</replaceable></term>
      <listitem>
       <para>
        The data type of the operator's right operand.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">com_op</replaceable></term>
      <listitem>
       <para>
        The commutator of this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">neg_op</replaceable></term>
      <listitem>
       <para>
        The negator of this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">res_proc</replaceable></term>
      <listitem>
       <para>
        The restriction selectivity estimator function for this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">join_proc</replaceable></term>
      <listitem>
       <para>
        The join selectivity estimator function for this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>HASHES</literal></term>
      <listitem>
       <para>
       Indicates this operator can support a hash join.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>MERGES</literal></term>
      <listitem>
       <para>
       Indicates this operator can support a merge join.
       </para>
      </listitem>
     </varlistentry>