326 lines
12 KiB
Plaintext
326 lines
12 KiB
Plaintext
<!-- doc/src/sgml/postgres-fdw.sgml -->
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<sect1 id="postgres-fdw" xreflabel="postgres_fdw">
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<title>postgres_fdw</title>
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<indexterm zone="postgres-fdw">
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<primary>postgres_fdw</primary>
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</indexterm>
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<para>
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The <filename>postgres_fdw</> module provides the foreign-data wrapper
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<literal>postgres_fdw</literal>, which can be used to access data
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stored in external <productname>PostgreSQL</productname> servers.
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</para>
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<para>
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The functionality provided by this module overlaps substantially
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with the functionality of the older <xref linkend="dblink"> module.
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But <filename>postgres_fdw</> provides more transparent and
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standards-compliant syntax for accessing remote tables, and can give
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better performance in many cases.
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</para>
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<para>
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To prepare for remote access using <filename>postgres_fdw</>:
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<orderedlist spacing="compact">
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<listitem>
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<para>
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Install the <filename>postgres_fdw</> extension using <xref
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linkend="sql-createextension">.
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</para>
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</listitem>
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<listitem>
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<para>
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Create a foreign server object, using <xref linkend="sql-createserver">,
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to represent each remote database you want to connect to.
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Specify connection information, except <literal>user</literal> and
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<literal>password</literal>, as options of the server object.
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</para>
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</listitem>
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<listitem>
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<para>
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Create a user mapping, using <xref linkend="sql-createusermapping">, for
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each database user you want to allow to access each foreign server.
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Specify the remote user name and password to use as
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<literal>user</literal> and <literal>password</literal> options of the
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user mapping.
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</para>
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</listitem>
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<listitem>
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<para>
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Create a foreign table, using <xref linkend="sql-createforeigntable">,
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for each remote table you want to access. The columns of the foreign
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table must match the referenced remote table. You can, however, use
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table and/or column names different from the remote table's, if you
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specify the correct remote names as options of the foreign table object.
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</para>
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</listitem>
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</orderedlist>
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</para>
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<para>
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Now you need only <command>SELECT</> from a foreign table to access
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the data stored in its underlying remote table.
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</para>
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<para>
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It is generally recommended that the columns of a foreign table be declared
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with exactly the same data types, and collations if applicable, as the
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referenced columns of the remote table. Although <filename>postgres_fdw</>
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is currently rather forgiving about performing data type conversions at
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need, surprising semantic anomalies may arise when types or collations do
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not match, due to the remote server interpreting <literal>WHERE</> clauses
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slightly differently from the local server.
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</para>
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<para>
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Note that a foreign table can be declared with fewer columns, or with a
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different column order, than its underlying remote table has. Matching
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of columns to the remote table is by name, not position.
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</para>
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<sect2>
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<title>FDW Options of postgres_fdw</title>
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<sect3>
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<title>Connection Options</title>
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<para>
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A foreign server using the <filename>postgres_fdw</> foreign data wrapper
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can have the same options that <application>libpq</> accepts in
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connection strings, as described in <xref linkend="libpq-paramkeywords">,
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except that these options are not allowed:
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<itemizedlist spacing="compact">
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<listitem>
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<para>
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<literal>user</literal> and <literal>password</literal> (specify these
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for a user mapping, instead)
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>client_encoding</> (this is automatically set from the local
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server encoding)
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>fallback_application_name</> (always set to
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<literal>postgres_fdw</>)
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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Only superusers may connect to foreign servers without password
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authentication, so always specify the <literal>password</literal> option
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for user mappings belonging to non-superusers.
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</para>
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</sect3>
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<sect3>
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<title>Object Name Options</title>
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<para>
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These options can be used to control the names used in SQL statements
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sent to the remote <productname>PostgreSQL</productname> server. These
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options are needed when a foreign table is created with names different
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from the underlying remote table's names.
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>schema_name</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign table, gives the
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schema name to use for the foreign table on the remote server. If this
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option is omitted, the name of the foreign table's schema is used.
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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>table_name</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign table, gives the
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table name to use for the foreign table on the remote server. If this
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option is omitted, the foreign table's name is used.
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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>column_name</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a column of a foreign table,
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gives the column name to use for the column on the remote server.
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If this option is omitted, the column's name is used.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect3>
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<sect3>
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<title>Cost Estimation Options</title>
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<para>
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<filename>postgres_fdw</> retrieves remote data by executing queries
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against remote servers, so ideally the estimated cost of scanning a
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foreign table should be whatever it costs to be done on the remote
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server, plus some overhead for communication. The most reliable way to
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get such an estimate is to ask the remote server and then add something
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for overhead — but for simple queries, it may not be worth the cost
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of an additional remote query to get a cost estimate.
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So <filename>postgres_fdw</> provides the following options to control
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how cost estimation is done:
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</para>
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<variablelist>
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<varlistentry>
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<term><literal>use_remote_estimate</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign table or a foreign
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server, controls whether <filename>postgres_fdw</> issues remote
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<command>EXPLAIN</command> commands to obtain cost estimates.
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A setting for a foreign table overrides any setting for its server,
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but only for that table.
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The default is <literal>false</literal>.
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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>fdw_startup_cost</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign server, is a numeric
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value that is added to the estimated startup cost of any foreign-table
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scan on that server. This represents the additional overhead of
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establishing a connection, parsing and planning the query on the
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remote side, etc.
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The default value is <literal>100</literal>.
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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>fdw_tuple_cost</literal></term>
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<listitem>
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<para>
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This option, which can be specified for a foreign server, is a numeric
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value that is used as extra cost per-tuple for foreign-table
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scans on that server. This represents the additional overhead of
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data transfer between servers. You might increase or decrease this
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number to reflect higher or lower network delay to the remote server.
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The default value is <literal>0.01</literal>.
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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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When <literal>use_remote_estimate</literal> is true,
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<filename>postgres_fdw</> obtains rowcount and cost estimates from the
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remote server and then adds <literal>fdw_startup_cost</literal> and
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<literal>fdw_tuple_cost</literal> to the cost estimates. When
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<literal>use_remote_estimate</literal> is false,
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<filename>postgres_fdw</> performs local rowcount and cost estimation
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and then adds <literal>fdw_startup_cost</literal> and
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<literal>fdw_tuple_cost</literal> to the cost estimates. This local
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estimation is unlikely to be very accurate unless local copies of the
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remote table's statistics are available. Running
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<xref linkend="sql-analyze"> on the foreign table is the way to update
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the local statistics; this will perform a scan of the remote table and
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then calculate and store statistics just as though the table were local.
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Keeping local statistics can be a useful way to reduce per-query planning
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overhead for a remote table — but if the remote table is
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frequently updated, the local statistics will soon be obsolete.
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</para>
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</sect3>
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</sect2>
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<sect2>
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<title>Connection Management</title>
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<para>
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<filename>postgres_fdw</filename> establishes a connection to a
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foreign server during the first query that uses a foreign table
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associated with the foreign server. This connection is kept and
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re-used for subsequent queries in the same session. However, if
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multiple user identities (user mappings) are used to access the foreign
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server, a connection is established for each user mapping.
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</para>
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</sect2>
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<sect2>
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<title>Transaction Management</title>
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<para>
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During a query that references any remote tables on a foreign server,
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<filename>postgres_fdw</filename> opens a transaction on the
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remote server if one is not already open corresponding to the current
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local transaction. The remote transaction is committed or aborted when
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the local transaction commits or aborts. Savepoints are similarly
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managed by creating corresponding remote savepoints.
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</para>
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<para>
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The remote transaction uses <literal>SERIALIZABLE</>
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isolation level when the local transaction has <literal>SERIALIZABLE</>
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isolation level; otherwise it uses <literal>REPEATABLE READ</>
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isolation level. This choice ensures that if a query performs multiple
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table scans on the remote server, it will get snapshot-consistent results
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for all the scans. A consequence is that successive queries within a
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single transaction will see the same data from the remote server, even if
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concurrent updates are occurring on the remote server due to other
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activities. That behavior would be expected anyway if the local
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transaction uses <literal>SERIALIZABLE</> or <literal>REPEATABLE READ</>
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isolation level, but it might be surprising for a <literal>READ
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COMMITTED</> local transaction. A future
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<productname>PostgreSQL</productname> release might modify these rules.
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</para>
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</sect2>
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<sect2>
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<title>Remote Query Optimization</title>
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<para>
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<filename>postgres_fdw</> attempts to optimize remote queries to reduce
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the amount of data transferred from foreign servers. This is done by
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sending query <literal>WHERE</> clauses to the remote server for
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execution, and by not retrieving table columns that are not needed for
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the current query. To reduce the risk of misexecution of queries,
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<literal>WHERE</> clauses are not sent to the remote server unless they use
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only built-in data types, operators, and functions. Operators and
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functions in the clauses must be <literal>IMMUTABLE</> as well.
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</para>
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<para>
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The query that is actually sent to the remote server for execution can
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be examined using <command>EXPLAIN VERBOSE</>.
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</para>
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</sect2>
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<sect2>
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<title>Author</title>
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<para>
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Shigeru Hanada <email>shigeru.hanada@gmail.com</email>
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</para>
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</sect2>
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</sect1>
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