3 files changed, 41 insertions, 14 deletions
diff --git a/doc/src/sgml/keywords.sgml b/doc/src/sgml/keywords.sgml
index 059c8e468f4..ecfde993da3 100644
--- a/doc/src/sgml/keywords.sgml
+++ b/doc/src/sgml/keywords.sgml
@@ -1786,7 +1786,7 @@
    </row>
    <row>
     <entry><token>FILTER</token></entry>
-    <entry></entry>
+    <entry>non-reserved</entry>
     <entry>reserved</entry>
     <entry>reserved</entry>
     <entry></entry>
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 68309ba94da..b0cec1421ca 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -598,6 +598,11 @@ GROUP BY <replaceable class="parameter">expression</replaceable> [, ...]
     making up each group, producing a separate value for each group
     (whereas without <literal>GROUP BY</literal>, an aggregate
     produces a single value computed across all the selected rows).
+    The set of rows fed to the aggregate function can be further filtered by
+    attaching a <literal>FILTER</literal> clause to the aggregate function
+    call; see <xref linkend="syntax-aggregates"> for more information.  When
+    a <literal>FILTER</literal> clause is present, only those rows matching it
+    are included.
     When <literal>GROUP BY</literal> is present, it is not valid for
     the <command>SELECT</command> list expressions to refer to
     ungrouped columns except within aggregate functions or if the
diff --git a/doc/src/sgml/syntax.sgml b/doc/src/sgml/syntax.sgml
index b1392124a9e..803ed855c82 100644
--- a/doc/src/sgml/syntax.sgml
+++ b/doc/src/sgml/syntax.sgml
@@ -1554,6 +1554,10 @@ sqrt(2)
     <secondary>invocation</secondary>
    </indexterm>
 
+   <indexterm zone="syntax-aggregates">
+    <primary>filter</primary>
+   </indexterm>
+
    <para>
     An <firstterm>aggregate expression</firstterm> represents the
     application of an aggregate function across the rows selected by a
@@ -1562,19 +1566,19 @@ sqrt(2)
     syntax of an aggregate expression is one of the following:
 
 <synopsis>
-<replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable> [ , ... ] [ <replaceable>order_by_clause</replaceable> ] )
-<replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable> [ , ... ] [ <replaceable>order_by_clause</replaceable> ] )
-<replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable> [ , ... ] [ <replaceable>order_by_clause</replaceable> ] )
-<replaceable>aggregate_name</replaceable> ( * )
+<replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable> [ , ... ] [ <replaceable>order_by_clause</replaceable> ] ) [ FILTER ( WHERE <replaceable>filter_clause</replaceable> ) ]
+<replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable> [ , ... ] [ <replaceable>order_by_clause</replaceable> ] ) [ FILTER ( WHERE <replaceable>filter_clause</replaceable> ) ]
+<replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable> [ , ... ] [ <replaceable>order_by_clause</replaceable> ] ) [ FILTER ( WHERE <replaceable>filter_clause</replaceable> ) ]
+<replaceable>aggregate_name</replaceable> ( * ) [ FILTER ( WHERE <replaceable>filter_clause</replaceable> ) ]
 </synopsis>
 
     where <replaceable>aggregate_name</replaceable> is a previously
-    defined aggregate (possibly qualified with a schema name),
+    defined aggregate (possibly qualified with a schema name) and
     <replaceable>expression</replaceable> is
     any value expression that does not itself contain an aggregate
-    expression or a window function call, and
-    <replaceable>order_by_clause</replaceable> is a optional
-    <literal>ORDER BY</> clause as described below.
+    expression or a window function call.  The optional
+    <replaceable>order_by_clause</replaceable> and
+    <replaceable>filter_clause</replaceable> are described below.
    </para>
 
    <para>
@@ -1607,6 +1611,23 @@ sqrt(2)
    </para>
 
    <para>
+    If <literal>FILTER</literal> is specified, then only the input
+    rows for which the <replaceable>filter_clause</replaceable>
+    evaluates to true are fed to the aggregate function; other rows
+    are discarded.  For example:
+<programlisting>
+SELECT
+    count(*) AS unfiltered,
+    count(*) FILTER (WHERE i < 5) AS filtered
+FROM generate_series(1,10) AS s(i);
+ unfiltered | filtered 
+------------+----------
+         10 |        4
+(1 row)
+</programlisting>
+   </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</> produces the same result no matter what order it
@@ -1709,10 +1730,10 @@ SELECT string_agg(a ORDER BY a, ',') FROM table;  -- incorrect
     The syntax of a window function call is one of the following:
 
 <synopsis>
-<replaceable>function_name</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional>) OVER ( <replaceable class="parameter">window_definition</replaceable> )
-<replaceable>function_name</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional>) OVER <replaceable>window_name</replaceable>
-<replaceable>function_name</replaceable> ( * ) OVER ( <replaceable class="parameter">window_definition</replaceable> )
-<replaceable>function_name</replaceable> ( * ) OVER <replaceable>window_name</replaceable>
+<replaceable>function_name</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional>) [ FILTER ( WHERE <replaceable>filter_clause</replaceable> ) ] OVER ( <replaceable class="parameter">window_definition</replaceable> )
+<replaceable>function_name</replaceable> (<optional><replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ... </optional></optional>) [ FILTER ( WHERE <replaceable>filter_clause</replaceable> ) ] OVER <replaceable>window_name</replaceable>
+<replaceable>function_name</replaceable> ( * ) [ FILTER ( WHERE <replaceable>filter_clause</replaceable> ) ] OVER ( <replaceable class="parameter">window_definition</replaceable> )
+<replaceable>function_name</replaceable> ( * ) [ FILTER ( WHERE <replaceable>filter_clause</replaceable> ) ] OVER <replaceable>window_name</replaceable>
 </synopsis>
     where <replaceable class="parameter">window_definition</replaceable>
     has the syntax
@@ -1836,7 +1857,8 @@ UNBOUNDED FOLLOWING
     The built-in window functions are described in <xref
     linkend="functions-window-table">.  Other window functions can be added by
     the user.  Also, any built-in or user-defined aggregate function can be
-    used as a window function.
+    used as a window function.  Only aggregate window functions accept
+    a <literal>FILTER</literal> clause.
    </para>
 
    <para>