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

<sect1 id="file-fdw" xreflabel="file_fdw">
 <title>file_fdw</title>

 <indexterm zone="file-fdw">
  <primary>file_fdw</primary>
 </indexterm>

 <para>
  The <filename>file_fdw</> module provides the foreign-data wrapper
  <function>file_fdw</function>, which can be used to access data
  files in the server's file system.  Data files must be in a format
  that can be read by <command>COPY FROM</command>;
  see <xref linkend="sql-copy"> for details.
  Access to such data files is currently read-only.
 </para>

 <para>
  A foreign table created using this wrapper can have the following options:
 </para>

 <variablelist>

  <varlistentry>
   <term><literal>filename</literal></term>

   <listitem>
    <para>
     Specifies the file to be read.  Required.  Must be an absolute path name.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>format</literal></term>

   <listitem>
    <para>
     Specifies the file's format,
     the same as <command>COPY</>'s <literal>FORMAT</literal> option.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>header</literal></term>

   <listitem>
    <para>
     Specifies whether the file has a header line,
     the same as <command>COPY</>'s <literal>HEADER</literal> option.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>delimiter</literal></term>

   <listitem>
    <para>
     Specifies the file's delimiter character,
     the same as <command>COPY</>'s <literal>DELIMITER</literal> option.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>quote</literal></term>

   <listitem>
    <para>
     Specifies the file's quote character,
     the same as <command>COPY</>'s <literal>QUOTE</literal> option.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>escape</literal></term>

   <listitem>
    <para>
     Specifies the file's escape character,
     the same as <command>COPY</>'s <literal>ESCAPE</literal> option.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>null</literal></term>

   <listitem>
    <para>
     Specifies the file's null string,
     the same as <command>COPY</>'s <literal>NULL</literal> option.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>encoding</literal></term>

   <listitem>
    <para>
     Specifies the file's encoding,
     the same as <command>COPY</>'s <literal>ENCODING</literal> option.
    </para>
   </listitem>
  </varlistentry>

 </variablelist>

 <para>
  Note that while <command>COPY</> allows options such as OIDS and HEADER
  to be specified without a corresponding value, the foreign data wrapper
  syntax requires a value to be present in all cases.  To activate
  <command>COPY</> options normally supplied without a value, you can
  instead pass the value TRUE.
 </para>

 <para>
  A column of a foreign table created using this wrapper can have the
  following options:
 </para>

 <variablelist>

  <varlistentry>
   <term><literal>force_not_null</literal></term>

   <listitem>
    <para>
     This is a Boolean option.  If true, it specifies that values of the
     column should not be matched against the null string (that is, the
     file-level <literal>null</literal> option).  This has the same effect
     as listing the column in <command>COPY</>'s
     <literal>FORCE_NOT_NULL</literal> option.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>force_null</literal></term>

   <listitem>
    <para>
     This is a Boolean option.  If true, it specifies that values of the
     column which match the null string are returned as <literal>NULL</>
     even if the value is quoted. Without this option, only unquoted
     values matching the null string are returned as <literal>NULL</>.
     This has the same effect  as listing the column in
     <command>COPY</>'s <literal>FORCE_NULL</literal> option.
    </para>
   </listitem>
  </varlistentry>

 </variablelist>

 <para>
  <command>COPY</>'s <literal>OIDS</literal> and
  <literal>FORCE_QUOTE</literal> options are currently not supported by
  <literal>file_fdw</>.
 </para>

 <para>
  These options can only be specified for a foreign table or its columns, not
  in the options of the <literal>file_fdw</> foreign-data wrapper, nor in the
  options of a server or user mapping using the wrapper.
 </para>

 <para>
  Changing table-level options requires superuser privileges, for security
  reasons: only a superuser should be able to determine which file is read.
  In principle non-superusers could be allowed to change the other options,
  but that's not supported at present.
 </para>

 <para>
  For a foreign table using <literal>file_fdw</>, <command>EXPLAIN</> shows
  the name of the file to be read.  Unless <literal>COSTS OFF</> is
  specified, the file size (in bytes) is shown as well.
 </para>

 <example>
 <title id="csvlog-fdw">Create a Foreign Table for PostgreSQL CSV Logs</title>

  <para>
   One of the obvious uses for the <literal>file_fdw</> is to make
   the PostgreSQL activity log available as a table for querying.  To
   do this, first you must be logging to a CSV file, which here we
   will call <literal>pglog.csv</>.  First, install <literal>file_fdw</>
   as an extension:
  </para>

<programlisting>
CREATE EXTENSION file_fdw;
</programlisting>

  <para>
   Then create a foreign server:

<programlisting>
CREATE SERVER pglog FOREIGN DATA WRAPPER file_fdw;
</programlisting>
  </para>

  <para>
   Now you are ready to create the foreign data table.  Using the
   <command>CREATE FOREIGN TABLE</> command, you will need to define
   the columns for the table, the CSV file name, and its format:

<programlisting>
CREATE FOREIGN TABLE pglog (
  log_time timestamp(3) with time zone,
  user_name text,
  database_name text,
  process_id integer,
  connection_from text,
  session_id text,
  session_line_num bigint,
  command_tag text,
  session_start_time timestamp with time zone,
  virtual_transaction_id text,
  transaction_id bigint,
  error_severity text,
  sql_state_code text,
  message text,
  detail text,
  hint text,
  internal_query text,
  internal_query_pos integer,
  context text,
  query text,
  query_pos integer,
  location text,
  application_name text
) SERVER pglog
OPTIONS ( filename '/home/josh/9.1/data/pg_log/pglog.csv', format 'csv' );
</programlisting>
  </para>

  <para>
   That's it &mdash; now you can query your log directly. In production, of
   course, you would need to define some way to deal with log rotation.
  </para>
 </example>

</sect1>
