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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>5.12. Foreign Data</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="ddl-partitioning.html" title="5.11. Table Partitioning" /><link rel="next" href="ddl-others.html" title="5.13. Other Database Objects" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.12. Foreign Data</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl-partitioning.html" title="5.11. Table Partitioning">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ddl-others.html" title="5.13. Other Database Objects">Next</a></td></tr></table><hr /></div><div class="sect1" id="DDL-FOREIGN-DATA"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.12. Foreign Data</h2></div></div></div><a id="id-1.5.4.14.2" class="indexterm"></a><a id="id-1.5.4.14.3" class="indexterm"></a><a id="id-1.5.4.14.4" class="indexterm"></a><p>
+    <span class="productname">PostgreSQL</span> implements portions of the SQL/MED
+    specification, allowing you to access data that resides outside
+    PostgreSQL using regular SQL queries.  Such data is referred to as
+    <em class="firstterm">foreign data</em>.  (Note that this usage is not to be confused
+    with foreign keys, which are a type of constraint within the database.)
+   </p><p>
+    Foreign data is accessed with help from a
+    <em class="firstterm">foreign data wrapper</em>. A foreign data wrapper is a
+    library that can communicate with an external data source, hiding the
+    details of connecting to the data source and obtaining data from it.
+    There are some foreign data wrappers available as <code class="filename">contrib</code>
+    modules; see <a class="xref" href="contrib.html" title="Appendix F. Additional Supplied Modules">Appendix F</a>.  Other kinds of foreign data
+    wrappers might be found as third party products.  If none of the existing
+    foreign data wrappers suit your needs, you can write your own; see <a class="xref" href="fdwhandler.html" title="Chapter 59. Writing a Foreign Data Wrapper">Chapter 59</a>.
+   </p><p>
+    To access foreign data, you need to create a <em class="firstterm">foreign server</em>
+    object, which defines how to connect to a particular external data source
+    according to the set of options used by its supporting foreign data
+    wrapper. Then you need to create one or more <em class="firstterm">foreign
+    tables</em>, which define the structure of the remote data. A
+    foreign table can be used in queries just like a normal table, but a
+    foreign table has no storage in the PostgreSQL server.  Whenever it is
+    used, <span class="productname">PostgreSQL</span> asks the foreign data wrapper
+    to fetch data from the external source, or transmit data to the external
+    source in the case of update commands.
+   </p><p>
+    Accessing remote data may require authenticating to the external
+    data source.  This information can be provided by a
+    <em class="firstterm">user mapping</em>, which can provide additional data
+    such as user names and passwords based
+    on the current <span class="productname">PostgreSQL</span> role.
+   </p><p>
+    For additional information, see
+    <a class="xref" href="sql-createforeigndatawrapper.html" title="CREATE FOREIGN DATA WRAPPER"><span class="refentrytitle">CREATE FOREIGN DATA WRAPPER</span></a>,
+    <a class="xref" href="sql-createserver.html" title="CREATE SERVER"><span class="refentrytitle">CREATE SERVER</span></a>,
+    <a class="xref" href="sql-createusermapping.html" title="CREATE USER MAPPING"><span class="refentrytitle">CREATE USER MAPPING</span></a>,
+    <a class="xref" href="sql-createforeigntable.html" title="CREATE FOREIGN TABLE"><span class="refentrytitle">CREATE FOREIGN TABLE</span></a>, and
+    <a class="xref" href="sql-importforeignschema.html" title="IMPORT FOREIGN SCHEMA"><span class="refentrytitle">IMPORT FOREIGN SCHEMA</span></a>.
+   </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl-partitioning.html" title="5.11. Table Partitioning">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-others.html" title="5.13. Other Database Objects">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.11. Table Partitioning </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 5.13. Other Database Objects</td></tr></table></div></body></html>
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