<quote>stand-alone</quote> composite type with no associated
     table.  A composite type is simply a list of types with
     associated field names.  A value of a composite type is a row or
     record of field values.  Refer to <xref linkend="rowtypes"/>
     for more information.
    </para>

    <para>
     A range type can hold two values of the same type, which are the lower
     and upper bounds of the range.  Range types are user-created, although
     a few built-in ones exist.  Refer to <xref linkend="rangetypes"/>
     for more information.
    </para>
   </sect2>

   <sect2 id="extend-type-system-domains">
    <title>Domains</title>

    <para>
     A domain is based on a particular underlying type and for many purposes
     is interchangeable with its underlying type.  However, a domain can have
     constraints that restrict its valid values to a subset of what the
     underlying type would allow.  Domains are created using
     the <acronym>SQL</acronym> command <xref linkend="sql-createdomain"/>.
     Refer to <xref linkend="domains"/> for more information.
    </para>
   </sect2>

   <sect2 id="extend-type-system-pseudo">
    <title>Pseudo-Types</title>

    <para>
     There are a few <quote>pseudo-types</quote> for special purposes.
     Pseudo-types cannot appear as columns of tables or components of
     container types, but they can be used to declare the argument and
     result types of functions.  This provides a mechanism within the
     type system to identify special classes of functions.  <xref
     linkend="datatype-pseudotypes-table"/> lists the existing
     pseudo-types.
    </para>
   </sect2>

   <sect2 id="extend-types-polymorphic">
    <title>Polymorphic Types</title>

   <indexterm zone="extend-types-polymorphic">
    <primary>polymorphic type</primary>
   </indexterm>

   <indexterm zone="extend-types-polymorphic">
    <primary>polymorphic function</primary>
   </indexterm>

   <indexterm zone="extend-types-polymorphic">
    <primary>data type</primary>
    <secondary>polymorphic</secondary>
   </indexterm>

   <indexterm zone="extend-types-polymorphic">
    <primary>function</primary>
    <secondary>polymorphic</secondary>
   </indexterm>

    <para>
     Some pseudo-types of special interest are the <firstterm>polymorphic
     types</firstterm>, which are used to declare <firstterm>polymorphic
     functions</firstterm>.  This powerful feature allows a single function
     definition to operate on many different data types, with the specific
     data type(s) being determined by the data types actually passed to it
     in a particular call.  The polymorphic types are shown in
     <xref linkend="extend-types-polymorphic-table"/>.  Some examples of
     their use appear in <xref linkend="xfunc-sql-polymorphic-functions"/>.
    </para>

    <table id="extend-types-polymorphic-table">
     <title>Polymorphic Types</title>
     <tgroup cols="3">
      <colspec colname="col1" colwidth="2*"/>
      <colspec colname="col2" colwidth="1*"/>
      <colspec colname="col3" colwidth="2*"/>
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Family</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><type>anyelement</type></entry>
        <entry>Simple</entry>
        <entry>Indicates that a function accepts any data type</entry>
       </row>

       <row>
        <entry><type>anyarray</type></entry>
        <entry>Simple</entry>
        <entry>Indicates that a function accepts any array data type</entry>
       </row>

       <row>
        <entry><type>anynonarray</type></entry>
        <entry>Simple</entry>
        <entry>Indicates that a function accepts any non-array data type</entry>
       </row>

       <row>
        <entry><type>anyenum</type></entry>
        <entry>Simple</entry>
        <entry>Indicates that a function accepts any enum data type
        (see <xref linkend="datatype-enum"/>)
        </entry>