<firstterm>foreign data wrapper</firstterm>. 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 fetching data from it. There
- are several foreign data wrappers available, which can for example read
- plain data files residing on the server, or connect to another PostgreSQL
- instance. If none of the existing foreign data wrappers suit your needs,
- you can write your own; see <xref linkend="fdwhandler">.
+ is a foreign data wrapper available as a <file>contrib</file> module,
+ which can read plain data files residing on the server. Other kind 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 <xref linkend="fdwhandler">.
</para>
<para>
are not needed, you should insert nulls in those column positions.
</para>
+ <para>
+ Note that <productname>PostgreSQL</productname>'s executor doesn't care
+ whether the rows returned violate the <literal>NOT NULL</literal>
+ constraints which were defined on the foreign table columns - but the
+ planner does care, and may optimize queries incorrectly if
+ <literal>NULL</> values are present in a column declared not to contain
+ them. If a <literal>NULL</> value is encountered when the user has
+ declared that none should be present, it may be appropriate to raise an
+ error (just as you would need to do in the case of a data type mismatch).
+ </para>
+
<para>
<programlisting>
void