doc/src/sgml/ref/create_foreign_data_wrapper.sgml - master - githubmirror/postgres

doc/src/sgml/ref/create_foreign_data_wrapper.sgml
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Jeff Davis
CREATE SUBSCRIPTION ... SERVER.
06 мар 2026, 19:27
06 мар 2026, 19:278185bb5
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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 ]
    [ CONNECTION <replaceable class="parameter">connection_function</replaceable> | NO CONNECTION ]
    [ OPTIONS ( <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>CONNECTION <replaceable class="parameter">connection_function</replaceable></literal></term>
    <listitem>
     <para>
      <replaceable class="parameter">connection_function</replaceable> is the
      name of a previously registered function that will be called to generate
      the postgres connection string when a foreign server is used as part of
      <xref linkend="sql-createsubscription"/>.  If no connection function or
      <literal>NO CONNECTION</literal> is specified, then servers using this
      foreign data wrapper cannot be used for <literal>CREATE
      SUBSCRIPTION</literal>.  The connection function must take three
      arguments: one of type <type>oid</type> for the user, one of type
      <type>oid</type> for the server, and an unused third argument of type
      <type>internal</type> (which prevents calling the function in other
      contexts).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>OPTIONS ( <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>
  </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>