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diff --git a/doc/src/sgml/file-fdw.sgml b/doc/src/sgml/file-fdw.sgml new file mode 100644 index 0000000..eefc6e7 --- /dev/null +++ b/doc/src/sgml/file-fdw.sgml @@ -0,0 +1,280 @@ +<!-- 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</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 default role <literal>pg_read_server_files</literal> (to use a filename) or + the default 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 +) SERVER pglog +OPTIONS ( filename 'log/pglog.csv', format 'csv' ); +</programlisting> + </para> + + <para> + That's it — 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> |