<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