result of aggregation over some subset of
      the input values, to produce a
      new <replaceable class="parameter">state_data_type</replaceable> that
      represents the result of aggregating over both sets of inputs.  This
      function can be thought of as
      an <replaceable class="parameter">sfunc</replaceable>, where instead of
      acting upon an individual input row and adding it to the running
      aggregate state, it adds another aggregate state to the running state.
     </para>

     <para>
      The <replaceable class="parameter">combinefunc</replaceable> must be
      declared as taking two arguments of
      the <replaceable class="parameter">state_data_type</replaceable> and
      returning a value of
      the <replaceable class="parameter">state_data_type</replaceable>.
      Optionally this function may be <quote>strict</quote>. In this case the
      function will not be called when either of the input states are null;
      the other state will be taken as the correct result.
     </para>

     <para>
      For aggregate functions
      whose <replaceable class="parameter">state_data_type</replaceable>
      is <type>internal</type>,
      the <replaceable class="parameter">combinefunc</replaceable> must not
      be strict. In this case
      the <replaceable class="parameter">combinefunc</replaceable> must
      ensure that null states are handled correctly and that the state being
      returned is properly stored in the aggregate memory context.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">serialfunc</replaceable></term>
    <listitem>
     <para>
      An aggregate function
      whose <replaceable class="parameter">state_data_type</replaceable>
      is <type>internal</type> can participate in parallel aggregation only if it
      has a <replaceable class="parameter">serialfunc</replaceable> function,
      which must serialize the aggregate state into a <type>bytea</type> value for
      transmission to another process.  This function must take a single
      argument of type <type>internal</type> and return type <type>bytea</type>.  A
      corresponding <replaceable class="parameter">deserialfunc</replaceable>
      is also required.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">deserialfunc</replaceable></term>
    <listitem>
     <para>
      Deserialize a previously serialized aggregate state back into
      <replaceable class="parameter">state_data_type</replaceable>. This
      function must take two arguments of types <type>bytea</type>
      and <type>internal</type>, and produce a result of type <type>internal</type>.
      (Note: the second, <type>internal</type> argument is unused, but is required
      for type safety reasons.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">initial_condition</replaceable></term>
    <listitem>
     <para>
      The initial setting for the state value.  This must be a string
      constant in the form accepted for the data type <replaceable
      class="parameter">state_data_type</replaceable>.  If not
      specified, the state value starts out null.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">msfunc</replaceable></term>
    <listitem>
     <para>
      The name of the forward state transition function to be called for each
      input row in moving-aggregate mode.  This is exactly like the regular
      transition function, except that its first argument and result are of
      type <replaceable>mstate_data_type</replaceable>, which might be different
      from <replaceable>state_data_type</replaceable>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">minvfunc</replaceable></term>
    <listitem>
     <para>
      The name of the inverse