expression that does not itself contain an aggregate
    expression or a window function call.  The optional
    <replaceable>order_by_clause</replaceable> and
    <replaceable>filter_clause</replaceable> are described below.
   </para>

   <para>
    The first form of aggregate expression invokes the aggregate
    once for each input row.
    The second form is the same as the first, since
    <literal>ALL</literal> is the default.
    The third form invokes the aggregate once for each distinct value
    of the expression (or distinct set of values, for multiple expressions)
    found in the input rows.
    The fourth form invokes the aggregate once for each input row; since no
    particular input value is specified, it is generally only useful
    for the <function>count(*)</function> aggregate function.
    The last form is used with <firstterm>ordered-set</firstterm> aggregate
    functions, which are described below.
   </para>

   <para>
    Most aggregate functions ignore null inputs, so that rows in which
    one or more of the expression(s) yield null are discarded.  This
    can be assumed to be true, unless otherwise specified, for all
    built-in aggregates.
   </para>

   <para>
    For example, <literal>count(*)</literal> yields the total number
    of input rows; <literal>count(f1)</literal> yields the number of
    input rows in which <literal>f1</literal> is non-null, since
    <function>count</function> ignores nulls; and
    <literal>count(distinct f1)</literal> yields the number of
    distinct non-null values of <literal>f1</literal>.
   </para>

   <para>
    Ordinarily, the input rows are fed to the aggregate function in an
    unspecified order.  In many cases this does not matter; for example,
    <function>min</function> produces the same result no matter what order it
    receives the inputs in.  However, some aggregate functions
    (such as <function>array_agg</function> and <function>string_agg</function>) produce
    results that depend on the ordering of the input rows.  When using
    such an aggregate, the optional <replaceable>order_by_clause</replaceable> can be
    used to specify the desired ordering.  The <replaceable>order_by_clause</replaceable>
    has the same syntax as for a query-level <literal>ORDER BY</literal> clause, as
    described in <xref linkend="queries-order"/>, except that its expressions
    are always just expressions and cannot be output-column names or numbers.
    For example:
<programlisting>
WITH vals (v) AS ( VALUES (1),(3),(4),(3),(2) )
SELECT array_agg(v ORDER BY v DESC) FROM vals;
  array_agg
-------------
 {4,3,3,2,1}
</programlisting>
    Since <type>jsonb</type> only keeps the last matching key, ordering
    of its keys can be significant:
<programlisting>
WITH vals (k, v) AS ( VALUES ('key0','1'), ('key1','3'), ('key1','2') )
SELECT jsonb_object_agg(k, v ORDER BY v) FROM vals;
      jsonb_object_agg
----------------------------
 {"key0": "1", "key1": "3"}
</programlisting>
   </para>

   <para>
    When dealing with multiple-argument aggregate functions, note that the
    <literal>ORDER BY</literal> clause goes after all the aggregate arguments.
    For example, write this:
<programlisting>
SELECT string_agg(a, ',' ORDER BY a) FROM table;
</programlisting>
    not this:
<programlisting>
SELECT string_agg(a ORDER BY a, ',') FROM table;  -- incorrect
</programlisting>
    The latter is syntactically valid, but it represents a call of a
    single-argument aggregate function with two <literal>ORDER BY</literal> keys
    (the second one being rather useless since it's a constant).
   </para>

   <para>
    If <literal>DISTINCT</literal> is specified with an
    <replaceable>order_by_clause</replaceable>, <literal>ORDER
    BY</literal> expressions can only reference columns in the
    <literal>DISTINCT</literal> list.  For example:
<programlisting>
WITH vals (v) AS ( VALUES (1),(3),(4),(3),(2) )
SELECT array_agg(DISTINCT v ORDER BY v DESC) FROM vals;
 array_agg