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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>57.1. Foreign Data Wrapper Functions</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="fdwhandler.html" title="Chapter 57. Writing A Foreign Data Wrapper" /><link rel="next" href="fdw-callbacks.html" title="57.2. Foreign Data Wrapper Callback Routines" /></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">57.1. Foreign Data Wrapper Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="fdwhandler.html" title="Chapter 57. Writing A Foreign Data Wrapper">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="fdwhandler.html" title="Chapter 57. Writing A Foreign Data Wrapper">Up</a></td><th width="60%" align="center">Chapter 57. Writing A Foreign Data Wrapper</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="fdw-callbacks.html" title="57.2. Foreign Data Wrapper Callback Routines">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="FDW-FUNCTIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">57.1. Foreign Data Wrapper Functions</h2></div></div></div><p>
The FDW author needs to implement a handler function, and optionally
a validator function. Both functions must be written in a compiled
language such as C, using the version-1 interface.
For details on C language calling conventions and dynamic loading,
see <a class="xref" href="xfunc-c.html" title="38.10. C-Language Functions">Section 38.10</a>.
</p><p>
The handler function simply returns a struct of function pointers to
callback functions that will be called by the planner, executor, and
various maintenance commands.
Most of the effort in writing an FDW is in implementing these callback
functions.
The handler function must be registered with
<span class="productname">PostgreSQL</span> as taking no arguments and
returning the special pseudo-type <code class="type">fdw_handler</code>. The
callback functions are plain C functions and are not visible or
callable at the SQL level. The callback functions are described in
<a class="xref" href="fdw-callbacks.html" title="57.2. Foreign Data Wrapper Callback Routines">Section 57.2</a>.
</p><p>
The validator function is responsible for validating options given in
<code class="command">CREATE</code> and <code class="command">ALTER</code> commands for its
foreign data wrapper, as well as foreign servers, user mappings, and
foreign tables using the wrapper.
The validator function must be registered as taking two arguments, a
text array containing the options to be validated, and an OID
representing the type of object the options are associated with (in
the form of the OID of the system catalog the object would be stored
in, either
<code class="literal">ForeignDataWrapperRelationId</code>,
<code class="literal">ForeignServerRelationId</code>,
<code class="literal">UserMappingRelationId</code>,
or <code class="literal">ForeignTableRelationId</code>).
If no validator function is supplied, options are not checked at object
creation time or object alteration time.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="fdwhandler.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="fdwhandler.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="fdw-callbacks.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 57. Writing A Foreign Data Wrapper </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 57.2. Foreign Data Wrapper Callback Routines</td></tr></table></div></body></html>
