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This patch adds the core-system infrastructure needed to support updates on foreign tables, and extends contrib/postgres_fdw to allow updates against remote Postgres servers. There's still a great deal of room for improvement in optimization of remote updates, but at least there's basic functionality there now. KaiGai Kohei, reviewed by Alexander Korotkov and Laurenz Albe, and rather heavily revised by Tom Lane.
227 lines
5.7 KiB
Plaintext
227 lines
5.7 KiB
Plaintext
<!-- doc/src/sgml/file-fdw.sgml -->
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<sect1 id="file-fdw" xreflabel="file_fdw">
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<title>file_fdw</title>
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<indexterm zone="file-fdw">
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<primary>file_fdw</primary>
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</indexterm>
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<para>
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The <filename>file_fdw</> module provides the foreign-data wrapper
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<function>file_fdw</function>, which can be used to access data
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files in the server's file system. Data files must be in a format
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that can be read by <command>COPY FROM</command>;
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see <xref linkend="sql-copy"> for details.
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Access to such data files is currently read-only.
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</para>
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<para>
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A foreign table created using this wrapper can have the following options:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>filename</literal></term>
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<listitem>
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<para>
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Specifies the file to be read. Required. Must be an absolute path name.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>format</literal></term>
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<listitem>
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<para>
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Specifies the file's format,
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the same as <command>COPY</>'s <literal>FORMAT</literal> option.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>header</literal></term>
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<listitem>
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<para>
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Specifies whether the file has a header line,
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the same as <command>COPY</>'s <literal>HEADER</literal> option.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>delimiter</literal></term>
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<listitem>
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<para>
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Specifies the file's delimiter character,
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the same as <command>COPY</>'s <literal>DELIMITER</literal> option.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>quote</literal></term>
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<listitem>
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<para>
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Specifies the file's quote character,
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the same as <command>COPY</>'s <literal>QUOTE</literal> option.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>escape</literal></term>
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<listitem>
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<para>
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Specifies the file's escape character,
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the same as <command>COPY</>'s <literal>ESCAPE</literal> option.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>null</literal></term>
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<listitem>
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<para>
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Specifies the file's null string,
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the same as <command>COPY</>'s <literal>NULL</literal> option.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>encoding</literal></term>
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<listitem>
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<para>
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Specifies the file's encoding,
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the same as <command>COPY</>'s <literal>ENCODING</literal> option.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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A column of a foreign table created using this wrapper can have the
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following options:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>force_not_null</literal></term>
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<listitem>
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<para>
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This is a Boolean option. If true, it specifies that values of the
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column should not be matched against the null string (that is, the
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file-level <literal>null</literal> option). This has the same effect
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as listing the column in <command>COPY</>'s
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<literal>FORCE_NOT_NULL</literal> option.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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<command>COPY</>'s <literal>OIDS</literal> and
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<literal>FORCE_QUOTE</literal> options are currently not supported by
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<literal>file_fdw</>.
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</para>
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<para>
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These options can only be specified for a foreign table or its columns, not
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in the options of the <literal>file_fdw</> foreign-data wrapper, nor in the
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options of a server or user mapping using the wrapper.
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</para>
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<para>
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Changing table-level options requires superuser privileges, for security
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reasons: only a superuser should be able to determine which file is read.
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In principle non-superusers could be allowed to change the other options,
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but that's not supported at present.
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</para>
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<para>
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For a foreign table using <literal>file_fdw</>, <command>EXPLAIN</> shows
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the name of the file to be read. Unless <literal>COSTS OFF</> is
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specified, the file size (in bytes) is shown as well.
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</para>
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<example>
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<title id="csvlog-fdw">Create a Foreign Table for PostgreSQL CSV Logs</title>
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<para>
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One of the obvious uses for the <literal>file_fdw</> is to make
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the PostgreSQL activity log available as a table for querying. To
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do this, first you must be logging to a CSV file, which here we
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will call <literal>pglog.csv</>. First, install <literal>file_fdw</>
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as an extension:
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</para>
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<programlisting>
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CREATE EXTENSION file_fdw;
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</programlisting>
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<para>
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Then create a foreign server:
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<programlisting>
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CREATE SERVER pglog FOREIGN DATA WRAPPER file_fdw;
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</programlisting>
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</para>
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<para>
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Now you are ready to create the foreign data table. Using the
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<command>CREATE FOREIGN TABLE</> command, you will need to define
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the columns for the table, the CSV file name, and its format:
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<programlisting>
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CREATE FOREIGN TABLE pglog (
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log_time timestamp(3) with time zone,
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user_name text,
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database_name text,
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process_id integer,
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connection_from text,
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session_id text,
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session_line_num bigint,
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command_tag text,
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session_start_time timestamp with time zone,
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virtual_transaction_id text,
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transaction_id bigint,
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error_severity text,
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sql_state_code text,
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message text,
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detail text,
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hint text,
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internal_query text,
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internal_query_pos integer,
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context text,
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query text,
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query_pos integer,
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location text,
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application_name text
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) SERVER pglog
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OPTIONS ( filename '/home/josh/9.1/data/pg_log/pglog.csv', format 'csv' );
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</programlisting>
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</para>
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<para>
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That's it — now you can query your log directly. In production, of
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course, you would need to define some way to deal with log rotation.
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</para>
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</example>
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</sect1>
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