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>5.10. Foreign Data</A
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>    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> implements portions of the SQL/MED
    specification, allowing you to access data that resides outside
    PostgreSQL using regular SQL queries.  Such data is referred to as
    <I
CLASS="FIRSTTERM"
>foreign data</I
>.  (Note that this usage is not to be confused
    with foreign keys, which are a type of constraint within the database.)
   </P
><P
>    Foreign data is accessed with help from a
    <I
CLASS="FIRSTTERM"
>foreign data wrapper</I
>. A foreign data wrapper is a
    library that can communicate with an external data source, hiding the
    details of connecting to the data source and obtaining data from it.
    There are some foreign data wrappers available as <TT
CLASS="FILENAME"
>contrib</TT
>
    modules; see <A
HREF="contrib.html"
>Appendix F</A
>.  Other kinds of foreign data
    wrappers might be found as third party products.  If none of the existing
    foreign data wrappers suit your needs, you can write your own; see <A
HREF="fdwhandler.html"
>Chapter 52</A
>.
   </P
><P
>    To access foreign data, you need to create a <I
CLASS="FIRSTTERM"
>foreign server</I
>
    object, which defines how to connect to a particular external data source
    according to the set of options used by its supporting foreign data
    wrapper. Then you need to create one or more <I
CLASS="FIRSTTERM"
>foreign
    tables</I
>, which define the structure of the remote data. A
    foreign table can be used in queries just like a normal table, but a
    foreign table has no storage in the PostgreSQL server.  Whenever it is
    used, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> asks the foreign data wrapper
    to fetch data from the external source, or transmit data to the external
    source in the case of update commands.
   </P
><P
>    Accessing remote data may require authenticating to the external
    data source.  This information can be provided by a
    <I
CLASS="FIRSTTERM"
>user mapping</I
>, which can provide additional data
    such as user names and passwords based
    on the current <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> role.
   </P
><P
>    For additional information, see
    <A
HREF="sql-createforeigndatawrapper.html"
>CREATE FOREIGN DATA WRAPPER</A
>,
    <A
HREF="sql-createserver.html"
>CREATE SERVER</A
>,
    <A
HREF="sql-createusermapping.html"
>CREATE USER MAPPING</A
>, and
    <A
HREF="sql-createforeigntable.html"
>CREATE FOREIGN TABLE</A
>.
   </P
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