<!--
doc/src/sgml/ref/create_foreign_data_wrapper.sgml
PostgreSQL documentation
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<refentry id="sql-createforeigndatawrapper">
 <indexterm zone="sql-createforeigndatawrapper">
  <primary>CREATE FOREIGN DATA WRAPPER</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE FOREIGN DATA WRAPPER</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE FOREIGN DATA WRAPPER</refname>
  <refpurpose>define a new foreign-data wrapper</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE FOREIGN DATA WRAPPER <replaceable class="parameter">name</replaceable>
    [ HANDLER <replaceable class="parameter">handler_function</replaceable> | NO HANDLER ]
    [ VALIDATOR <replaceable class="parameter">validator_function</replaceable> | NO VALIDATOR ]
    [ OPTIONS ( [ mpp_execute 'any | master | all segments' ], <replaceable class="parameter">option</replaceable> '<replaceable class="parameter">value</replaceable>' [, ... ] ) ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>CREATE FOREIGN DATA WRAPPER</command> creates a new
   foreign-data wrapper.  The user who defines a foreign-data wrapper
   becomes its owner.
  </para>

  <para>
   The foreign-data wrapper name must be unique within the database.
  </para>

  <para>
   Only superusers can create foreign-data wrappers.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">name</replaceable></term>
    <listitem>
     <para>
      The name of the foreign-data wrapper to be created.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>HANDLER <replaceable class="parameter">handler_function</replaceable></literal></term>
    <listitem>
     <para><replaceable class="parameter">handler_function</replaceable> is the
      name of a previously registered function that will be called to
      retrieve the execution functions for foreign tables.
      The handler function must take no arguments, and
      its return type must be <type>fdw_handler</type>.
     </para>

     <para>
      It is possible to create a foreign-data wrapper with no handler
      function, but foreign tables using such a wrapper can only be declared,
      not accessed.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>VALIDATOR <replaceable class="parameter">validator_function</replaceable></literal></term>
    <listitem>
     <para><replaceable class="parameter">validator_function</replaceable>
      is the name of a previously registered function that will be called to
      check the generic options given to the foreign-data wrapper, as
      well as options for foreign servers, user mappings and foreign tables
      using the foreign-data wrapper.  If no validator function or <literal>NO
      VALIDATOR</literal> is specified, then options will not be
      checked at creation time.  (Foreign-data wrappers will possibly
      ignore or reject invalid option specifications at run time,
      depending on the implementation.)  The validator function must
      take two arguments: one of type <type>text[]</type>, which will
      contain the array of options as stored in the system catalogs,
      and one of type <type>oid</type>, which will be the OID of the
      system catalog containing the options. The return type is ignored;
      the function should report invalid options using the
      <function>ereport(ERROR)</function> function.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>OPTIONS ( [ mpp_execute 'any | master | all segments' ], <replaceable class="parameter">option</replaceable> '<replaceable class="parameter">value</replaceable>' [, ... ] )</literal></term>
    <listitem>
     <para>
      This clause specifies options for the new foreign-data wrapper.
      The allowed option names and values are specific to each foreign
      data wrapper and are validated using the foreign-data wrapper's
      validator function.  Option names must be unique.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">mpp_execute</replaceable></term>
    <listitem>
     <para>
     MASTER is the default, FDW requests for data from master.

     ANY, FDW requests for data from master or one any segment, depends on
     which path costs less.

     ALL SEGMENTS, FDW requests for data from all segments, wrappers need to
     have a policy matching the segments to data.

     A mpp_execute option setting specified by this FDW's foreign server
     takes precedence.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   <productname>PostgreSQL</productname>'s foreign-data functionality is still under
   active development.  Optimization of queries is primitive (and mostly left
   to the wrapper, too).  Thus, there is considerable room for future
   performance improvements.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Create a useless foreign-data wrapper <literal>dummy</literal>:
<programlisting>
CREATE FOREIGN DATA WRAPPER dummy;
</programlisting>
  </para>

  <para>
   Create a foreign-data wrapper <literal>file</literal> with
   handler function <literal>file_fdw_handler</literal>:
<programlisting>
CREATE FOREIGN DATA WRAPPER file HANDLER file_fdw_handler;
</programlisting>
  </para>

  <para>
   Create a foreign-data wrapper <literal>mywrapper</literal> with some
   options:
<programlisting>
CREATE FOREIGN DATA WRAPPER mywrapper
    OPTIONS (debug 'true');
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   <command>CREATE FOREIGN DATA WRAPPER</command> conforms to ISO/IEC
   9075-9 (SQL/MED), with the exception that the <literal>HANDLER</literal>
   and <literal>VALIDATOR</literal> clauses are extensions and the standard
   clauses <literal>LIBRARY</literal> and <literal>LANGUAGE</literal>
   are not implemented in <productname>PostgreSQL</productname>.
  </para>

  <para>
   Note, however, that the SQL/MED functionality as a whole is not yet
   conforming.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-alterforeigndatawrapper"/></member>
   <member><xref linkend="sql-dropforeigndatawrapper"/></member>
   <member><xref linkend="sql-createserver"/></member>
   <member><xref linkend="sql-createusermapping"/></member>
   <member><xref linkend="sql-createforeigntable"/></member>
  </simplelist>
 </refsect1>

</refentry>
