<literal>FILTER</literal> clause.
   </para>

   <para>
    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 general-purpose or
    statistical aggregate can be used as a window function.  (Ordered-set
    and hypothetical-set aggregates cannot presently be used as window functions.)
   </para>

   <para>
    The syntaxes using <literal>*</literal> are used for calling parameter-less
    aggregate functions as window functions, for example
    <literal>count(*) OVER (PARTITION BY x ORDER BY y)</literal>.
    The asterisk (<literal>*</literal>) is customarily not used for
    window-specific functions.  Window-specific functions do not
    allow <literal>DISTINCT</literal> or <literal>ORDER BY</literal> to be used within the
    function argument list.
   </para>

   <para>
    Window function calls are permitted only in the <literal>SELECT</literal>
    list and the <literal>ORDER BY</literal> clause of the query.
   </para>

   <para>
    More information about window functions can be found in
    <xref linkend="tutorial-window"/>,
    <xref linkend="functions-window"/>, and
    <xref linkend="queries-window"/>.
   </para>
  </sect2>

  <sect2 id="sql-syntax-type-casts">
   <title>Type Casts</title>

   <indexterm>
    <primary>data type</primary>
    <secondary>type cast</secondary>
   </indexterm>

   <indexterm>
    <primary>type cast</primary>
   </indexterm>

   <indexterm>
    <primary>::</primary>
   </indexterm>

   <para>
    A type cast specifies a conversion from one data type to another.
    <productname>PostgreSQL</productname> accepts two equivalent syntaxes
    for type casts:
<synopsis>
CAST ( <replaceable>expression</replaceable> AS <replaceable>type</replaceable> )
<replaceable>expression</replaceable>::<replaceable>type</replaceable>
</synopsis>
    The <literal>CAST</literal> syntax conforms to SQL; the syntax with
    <literal>::</literal> is historical <productname>PostgreSQL</productname>
    usage.
   </para>

   <para>
    When a cast is applied to a value expression of a known type, it
    represents a run-time type conversion.  The cast will succeed only
    if a suitable type conversion operation has been defined.  Notice that this
    is subtly different from the use of casts with constants, as shown in
    <xref linkend="sql-syntax-constants-generic"/>.  A cast applied to an
    unadorned string literal represents the initial assignment of a type
    to a literal constant value, and so it will succeed for any type
    (if the contents of the string literal are acceptable input syntax for the
    data type).
   </para>

   <para>
    An explicit type cast can usually be omitted if there is no ambiguity as
    to the type that a value expression must produce (for example, when it is
    assigned to a table column); the system will automatically apply a
    type cast in such cases.  However, automatic casting is only done for
    casts that are marked <quote>OK to apply implicitly</quote>
    in the system catalogs.  Other casts must be invoked with
    explicit casting syntax.  This restriction is intended to prevent
    surprising conversions from being applied silently.
   </para>

   <para>
    It is also possible to specify a type cast using a function-like
    syntax:
<synopsis>
<replaceable>typename</replaceable> ( <replaceable>expression</replaceable> )
</synopsis>
    However, this only works for types whose names are also valid as
    function names.  For example, <literal>double precision</literal>
    cannot be used this way, but the equivalent <literal>float8</literal>
    can.  Also, the names <literal>interval</literal>, <literal>time</literal>, and
    <literal>timestamp</literal> can only be used in this fashion if they are
    double-quoted, because of syntactic conflicts.  Therefore, the use of
    the function-like cast syntax leads to inconsistencies