<quote>on the
    fly</quote> makes <productname>PostgreSQL</productname> uniquely
    suited for rapid prototyping of new applications and storage
    structures.
   </para>
  </sect1>

  <sect1 id="extend-type-system">
   <title>The <productname>PostgreSQL</productname> Type System</title>

   <indexterm zone="extend-type-system">
    <primary>base type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>base</secondary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>composite type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>composite</secondary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>container type</primary>
   </indexterm>

   <indexterm zone="extend-type-system">
    <primary>data type</primary>
    <secondary>container</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname> data types can be divided into base
    types, container types, domains, and pseudo-types.
   </para>

   <sect2 id="extend-type-system-base">
    <title>Base Types</title>

    <para>
     Base types are those, like <type>integer</type>, that are
     implemented below the level of the <acronym>SQL</acronym> language
     (typically in a low-level language such as C).  They generally
     correspond to what are often known as abstract data types.
     <productname>PostgreSQL</productname> can only operate on such
     types through functions provided by the user and only understands
     the behavior of such types to the extent that the user describes
     them.
     The built-in base types are described in <xref linkend="datatype"/>.
    </para>

    <para>
     Enumerated (enum) types can be considered as a subcategory of base
     types.  The main difference is that they can be created using
     just <acronym>SQL</acronym> commands, without any low-level programming.
     Refer to <xref linkend="datatype-enum"/> for more information.
    </para>
   </sect2>

   <sect2 id="extend-type-system-container">
    <title>Container Types</title>

    <para>
     <productname>PostgreSQL</productname> has three kinds
     of <quote>container</quote> types, which are types that contain multiple
     values of other types.  These are arrays, composites, and ranges.
    </para>

    <para>
     Arrays can hold multiple values that are all of the same type.  An array
     type is automatically created for each base type, composite type, range
     type, and domain type.  But there are no arrays of arrays.  So far as
     the type system is concerned, multi-dimensional arrays are the same as
     one-dimensional arrays.  Refer to <xref linkend="arrays"/> for more
     information.
    </para>

    <para>
     Composite types, or row types, are created whenever the user
     creates a table. It is also possible to use <xref
     linkend="sql-createtype"/> to
     define a <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