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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 V1.79.1" /><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><div xmlns="http://www.w3.org/TR/xhtml1/transitional" 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 12.4 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></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 56. Writing a Foreign Data Wrapper">Chapter 56</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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-others.html">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">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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