<!-- doc/src/sgml/postgres-fdw.sgml -->

<sect1 id="postgres-fdw" xreflabel="postgres_fdw">
 <title>postgres_fdw</title>

 <indexterm zone="postgres-fdw">
  <primary>postgres_fdw</primary>
 </indexterm>

 <para>
  The <filename>postgres_fdw</> module provides the foreign-data wrapper
  <literal>postgres_fdw</literal>, which can be used to access data
  stored in external <productname>PostgreSQL</productname> servers.
 </para>

 <para>
  The functionality provided by this module overlaps substantially
  with the functionality of the older <xref linkend="dblink"> module.
  But <filename>postgres_fdw</> provides more transparent and
  standards-compliant syntax for accessing remote tables, and can give
  better performance in many cases.
 </para>

 <para>
  To prepare for remote access using <filename>postgres_fdw</>:
  <orderedlist spacing="compact">
   <listitem>
    <para>
     Install the  <filename>postgres_fdw</> extension using <xref
     linkend="sql-createextension">.
    </para>
   </listitem>
   <listitem>
    <para>
     Create a foreign server object, using <xref linkend="sql-createserver">,
     to represent each remote database you want to connect to.
     Specify connection information, except <literal>user</literal> and
     <literal>password</literal>, as options of the server object.
    </para>
   </listitem>
   <listitem>
    <para>
     Create a user mapping, using <xref linkend="sql-createusermapping">, for
     each database user you want to allow to access each foreign server.
     Specify the remote user name and password to use as
     <literal>user</literal> and <literal>password</literal> options of the
     user mapping.
    </para>
   </listitem>
   <listitem>
    <para>
     Create a foreign table, using <xref linkend="sql-createforeigntable">,
     for each remote table you want to access.  The columns of the foreign
     table must match the referenced remote table.  You can, however, use
     table and/or column names different from the remote table's, if you
     specify the correct remote names as options of the foreign table object.
    </para>
   </listitem>
  </orderedlist>
 </para>

 <para>
  Now you need only <command>SELECT</> from a foreign table to access
  the data stored in its underlying remote table.  You can also modify
  the remote table using <command>INSERT</>, <command>UPDATE</>, or
  <command>DELETE</>.  (Of course, the remote user you have specified
  in your user mapping must have privileges to do these things.)
 </para>

 <para>
  It is generally recommended that the columns of a foreign table be declared
  with exactly the same data types, and collations if applicable, as the
  referenced columns of the remote table.  Although <filename>postgres_fdw</>
  is currently rather forgiving about performing data type conversions at
  need, surprising semantic anomalies may arise when types or collations do
  not match, due to the remote server interpreting <literal>WHERE</> clauses
  slightly differently from the local server.
 </para>

 <para>
  Note that a foreign table can be declared with fewer columns, or with a
  different column order, than its underlying remote table has.  Matching
  of columns to the remote table is by name, not position.
 </para>

 <sect2>
  <title>FDW Options of postgres_fdw</title>

  <sect3>
   <title>Connection Options</title>

   <para>
    A foreign server using the <filename>postgres_fdw</> foreign data wrapper
    can have the same options that <application>libpq</> accepts in
    connection strings, as described in <xref linkend="libpq-paramkeywords">,
    except that these options are not allowed:

    <itemizedlist spacing="compact">
     <listitem>
      <para>
       <literal>user</literal> and <literal>password</literal> (specify these
       for a user mapping, instead)
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>client_encoding</> (this is automatically set from the local
       server encoding)
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>fallback_application_name</> (always set to
       <literal>postgres_fdw</>)
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    Only superusers may connect to foreign servers without password
    authentication, so always specify the <literal>password</literal> option
    for user mappings belonging to non-superusers.
   </para>
  </sect3>

  <sect3>
   <title>Object Name Options</title>

   <para>
    These options can be used to control the names used in SQL statements
    sent to the remote <productname>PostgreSQL</productname> server.  These
    options are needed when a foreign table is created with names different
    from the underlying remote table's names.
   </para>

   <variablelist>

    <varlistentry>
     <term><literal>schema_name</literal></term>
     <listitem>
      <para>
       This option, which can be specified for a foreign table, gives the
       schema name to use for the foreign table on the remote server.  If this
       option is omitted, the name of the foreign table's schema is used.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>table_name</literal></term>
     <listitem>
      <para>
       This option, which can be specified for a foreign table, gives the
       table name to use for the foreign table on the remote server.  If this
       option is omitted, the foreign table's name is used.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>column_name</literal></term>
     <listitem>
      <para>
       This option, which can be specified for a column of a foreign table,
       gives the column name to use for the column on the remote server.
       If this option is omitted, the column's name is used.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>

  </sect3>

  <sect3>
   <title>Cost Estimation Options</title>

   <para>
    <filename>postgres_fdw</> retrieves remote data by executing queries
    against remote servers, so ideally the estimated cost of scanning a
    foreign table should be whatever it costs to be done on the remote
    server, plus some overhead for communication.  The most reliable way to
    get such an estimate is to ask the remote server and then add something
    for overhead &mdash; but for simple queries, it may not be worth the cost
    of an additional remote query to get a cost estimate.
    So <filename>postgres_fdw</> provides the following options to control
    how cost estimation is done:
   </para>

   <variablelist>

    <varlistentry>
     <term><literal>use_remote_estimate</literal></term>
     <listitem>
      <para>
       This option, which can be specified for a foreign table or a foreign
       server, controls whether <filename>postgres_fdw</> issues remote
       <command>EXPLAIN</command> commands to obtain cost estimates.
       A setting for a foreign table overrides any setting for its server,
       but only for that table.
       The default is <literal>false</literal>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>fdw_startup_cost</literal></term>
     <listitem>
      <para>
       This option, which can be specified for a foreign server, is a numeric
       value that is added to the estimated startup cost of any foreign-table
       scan on that server.  This represents the additional overhead of
       establishing a connection, parsing and planning the query on the
       remote side, etc.
       The default value is <literal>100</literal>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>fdw_tuple_cost</literal></term>
     <listitem>
      <para>
       This option, which can be specified for a foreign server, is a numeric
       value that is used as extra cost per-tuple for foreign-table
       scans on that server.  This represents the additional overhead of
       data transfer between servers.  You might increase or decrease this
       number to reflect higher or lower network delay to the remote server.
       The default value is <literal>0.01</literal>.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>

   <para>
    When <literal>use_remote_estimate</literal> is true,
    <filename>postgres_fdw</> obtains row count and cost estimates from the
    remote server and then adds <literal>fdw_startup_cost</literal> and
    <literal>fdw_tuple_cost</literal> to the cost estimates.  When
    <literal>use_remote_estimate</literal> is false,
    <filename>postgres_fdw</> performs local row count and cost estimation
    and then adds <literal>fdw_startup_cost</literal> and
    <literal>fdw_tuple_cost</literal> to the cost estimates.  This local
    estimation is unlikely to be very accurate unless local copies of the
    remote table's statistics are available.  Running
    <xref linkend="sql-analyze"> on the foreign table is the way to update
    the local statistics; this will perform a scan of the remote table and
    then calculate and store statistics just as though the table were local.
    Keeping local statistics can be a useful way to reduce per-query planning
    overhead for a remote table &mdash; but if the remote table is
    frequently updated, the local statistics will soon be obsolete.
   </para>

  </sect3>

  <sect3>
   <title>Updatability Options</title>

   <para>
    By default all foreign tables using <filename>postgres_fdw</> are assumed
    to be updatable.  This may be overridden using the following option:
   </para>

   <variablelist>

    <varlistentry>
     <term><literal>updatable</literal></term>
     <listitem>
      <para>
       This option controls whether <filename>postgres_fdw</> allows foreign
       tables to be modified using <command>INSERT</>, <command>UPDATE</> and
       <command>DELETE</> commands.  It can be specified for a foreign table
       or a foreign server.  A table-level option overrides a server-level
       option.
       The default is <literal>true</>.
      </para>

      <para>
       Of course, if the remote table is not in fact updatable, an error
       would occur anyway.  Use of this option primarily allows the error to
       be thrown locally without querying the remote server.  Note however
       that the <literal>information_schema</> views will report a
       <filename>postgres_fdw</> foreign table to be updatable (or not)
       according to the setting of this option, without any check of the
       remote server.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </sect3>
 </sect2>

 <sect2>
  <title>Connection Management</title>

  <para>
   <filename>postgres_fdw</filename> establishes a connection to a
   foreign server during the first query that uses a foreign table
   associated with the foreign server.  This connection is kept and
   re-used for subsequent queries in the same session.  However, if
   multiple user identities (user mappings) are used to access the foreign
   server, a connection is established for each user mapping.
  </para>
 </sect2>

 <sect2>
  <title>Transaction Management</title>

  <para>
   During a query that references any remote tables on a foreign server,
   <filename>postgres_fdw</filename> opens a transaction on the
   remote server if one is not already open corresponding to the current
   local transaction.  The remote transaction is committed or aborted when
   the local transaction commits or aborts.  Savepoints are similarly
   managed by creating corresponding remote savepoints.
  </para>

  <para>
   The remote transaction uses <literal>SERIALIZABLE</>
   isolation level when the local transaction has <literal>SERIALIZABLE</>
   isolation level; otherwise it uses <literal>REPEATABLE READ</>
   isolation level.  This choice ensures that if a query performs multiple
   table scans on the remote server, it will get snapshot-consistent results
   for all the scans.  A consequence is that successive queries within a
   single transaction will see the same data from the remote server, even if
   concurrent updates are occurring on the remote server due to other
   activities.  That behavior would be expected anyway if the local
   transaction uses <literal>SERIALIZABLE</> or <literal>REPEATABLE READ</>
   isolation level, but it might be surprising for a <literal>READ
   COMMITTED</> local transaction.  A future
   <productname>PostgreSQL</productname> release might modify these rules.
  </para>
 </sect2>

 <sect2>
  <title>Remote Query Optimization</title>

  <para>
   <filename>postgres_fdw</> attempts to optimize remote queries to reduce
   the amount of data transferred from foreign servers.  This is done by
   sending query <literal>WHERE</> clauses to the remote server for
   execution, and by not retrieving table columns that are not needed for
   the current query.  To reduce the risk of misexecution of queries,
   <literal>WHERE</> clauses are not sent to the remote server unless they use
   only built-in data types, operators, and functions.  Operators and
   functions in the clauses must be <literal>IMMUTABLE</> as well.
  </para>

  <para>
   The query that is actually sent to the remote server for execution can
   be examined using <command>EXPLAIN VERBOSE</>.
  </para>
 </sect2>

 <sect2>
  <title>Remote Query Execution Environment</title>

  <para>
   In the remote sessions opened by <filename>postgres_fdw</>,
   the <xref linkend="guc-search-path"> parameter is set to
   just <literal>pg_catalog</>, so that only built-in objects are visible
   without schema qualification.  This is not an issue for queries
   generated by <filename>postgres_fdw</> itself, because it always
   supplies such qualification.  However, this can pose a hazard for
   functions that are executed on the remote server via triggers or rules
   on remote tables.  For example, if a remote table is actually a view,
   any functions used in that view will be executed with the restricted
   search path.  It is recommended to schema-qualify all names in such
   functions, or else attach <literal>SET search_path</> options
   (see <xref linkend="sql-createfunction">) to such functions
   to establish their expected search path environment.
  </para>

  <para>
   <filename>postgres_fdw</> likewise establishes remote session settings
   for the parameters <xref linkend="guc-timezone">,
   <xref linkend="guc-datestyle">, <xref linkend="guc-intervalstyle">,
   and <xref linkend="guc-extra-float-digits">.  These are less likely
   to be problematic than <varname>search_path</>, but can be handled
   with function <literal>SET</> options if the need arises.
  </para>

  <para>
   It is <emphasis>not</> recommended that you override this behavior by
   changing the session-level settings of these parameters; that is likely
   to cause <filename>postgres_fdw</> to malfunction.
  </para>
 </sect2>

 <sect2>
  <title>Cross-Version Compatibility</title>

  <para>
   <filename>postgres_fdw</> can be used with remote servers dating back
   to <productname>PostgreSQL</> 8.3.  Read-only capability is available
   back to 8.1.  A limitation however is that <filename>postgres_fdw</>
   generally assumes that immutable built-in functions and operators are
   safe to send to the remote server for execution, if they appear in a
   <literal>WHERE</> clause for a foreign table.  Thus, a built-in
   function that was added since the remote server's release might be sent
   to it for execution, resulting in <quote>function does not exist</> or
   a similar error.  This type of failure can be worked around by
   rewriting the query, for example by embedding the foreign table
   reference in a sub-<literal>SELECT</> with <literal>OFFSET 0</> as an
   optimization fence, and placing the problematic function or operator
   outside the sub-<literal>SELECT</>.
  </para>
 </sect2>

 <sect2>
  <title>Author</title>
  <para>
   Shigeru Hanada <email>shigeru.hanada@gmail.com</email>
  </para>
 </sect2>

</sect1>
