<indexterm>

<primary>foreign table</primary>

</indexterm>

<indexterm>

<primary>user mapping</primary>

</indexterm>

<para>

<productname>PostgreSQL</productname> implements portions of the SQL/MED

tables</firstterm>, 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, PostgreSQL asks the foreign data wrapper to fetch the data from the

external source.

used, <productname>PostgreSQL</productname> asks the foreign data wrapper

to fetch the data from the external source.

</para>

<para>

Accessing remote data may require authentication at the external

data source. This information can be provided by a

<firstterm>user mapping</>, which can provide additional options based

on the current <productname>PostgreSQL</productname> role.

</para>

<para>

</programlisting>

Begin executing a foreign scan. This is called during executor startup.

It should perform any initialization needed before the scan can start.

It should perform any initialization needed before the scan can start,

but not start executing the actual scan (that should be done upon the

first call to <function>IterateForeignScan</>).

The <structname>ForeignScanState</> node has already been created, but

its <structfield>fdw_state</> field is still NULL. Information about

the table to scan is accessible through the