postgres/doc/src/sgml/file-fdw.sgml

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

<sect1 id="file-fdw" xreflabel="file_fdw">
 <title>file_fdw &mdash; access data files in the server's file system</title>

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

 <para>
  The <filename>file_fdw</filename> module provides the foreign-data wrapper
  <function>file_fdw</function>, which can be used to access data
  files in the server's file system, or to execute programs on the server
  and read their output.  The data file or program output must be in a format
  that can be read by <command>COPY FROM</command>;
  see <xref linkend="sql-copy"/> for details.
  Access to 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.  Relative paths are relative to the
     data directory.
     Either <literal>filename</literal> or <literal>program</literal> must be
     specified, but not both.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry>
   <term><literal>program</literal></term>

   <listitem>
    <para>
     Specifies the command to be executed.  The standard output of this
     command will be read as though <command>COPY FROM PROGRAM</command> were used.
     Either <literal>program</literal> or <literal>filename</literal> must be
     specified, but not both.
    </para>
   </listitem>
  </varlistentry>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 </variablelist>

 <para>
  Note that while <command>COPY</command> allows options such as <literal>HEADER</literal>
  to be specified without a corresponding value, the foreign table option
  syntax requires a value to be present in all cases.  To activate
  <command>COPY</command> options typically written without a value, you can pass
  the value TRUE, since all such options are Booleans.
 </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
     table-level <literal>null</literal> option).  This has the same effect
     as listing the column in <command>COPY</command>'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</literal>
     even if the value is quoted. Without this option, only unquoted
     values matching the null string are returned as <literal>NULL</literal>.
     This has the same effect  as listing the column in
     <command>COPY</command>'s <literal>FORCE_NULL</literal> option.
    </para>
   </listitem>
  </varlistentry>

 </variablelist>

 <para>
  <command>COPY</command>'s <literal>FORCE_QUOTE</literal> option is
  currently not supported by <literal>file_fdw</literal>.
 </para>

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

 <para>
  Changing table-level options requires being a superuser or having the privileges
  of the role <literal>pg_read_server_files</literal> (to use a filename) or
  the role <literal>pg_execute_server_program</literal> (to use a program),
  for security reasons: only certain users should be able to control which file is
  read or which program is run.  In principle regular users could be allowed to
  change the other options, but that's not supported at present.
 </para>

 <para>
  When specifying the <literal>program</literal> option, keep in mind that the option
  string is executed by the shell.  If you need to pass any arguments to the
  command that come from an untrusted source, you must be careful to strip or
  escape any characters that might have special meaning to the shell.
  For security reasons, it is best to use a fixed command string, or at least
  avoid passing any user input in it.
 </para>

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

 <example>
  <title>Create a Foreign Table for PostgreSQL CSV Logs</title>

  <para>
   One of the obvious uses for <literal>file_fdw</literal> is to make
   the PostgreSQL activity log available as a table for querying.  To
   do this, first you must be <link
   linkend="runtime-config-logging-csvlog">logging to a CSV file,</link>
   which here we
   will call <literal>pglog.csv</literal>.  First, install <literal>file_fdw</literal>
   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> 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,
  backend_type text,
  leader_pid integer,
  query_id bigint
) SERVER pglog
OPTIONS ( filename '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>