and most foreign data wrappers
do not attempt to enforce them either; that is, the constraint is
simply assumed to hold true. There would be little point in such
enforcement since it would only apply to rows inserted or updated via
the foreign table, and not to rows modified by other means, such as
directly on the remote server. Instead, a constraint attached to a
foreign table should represent a constraint that is being enforced by
the remote server.
</para>
<para>
Some special-purpose foreign data wrappers might be the only access
mechanism for the data they access, and in that case it might be
appropriate for the foreign data wrapper itself to perform constraint
enforcement. But you should not assume that a wrapper does that
unless its documentation says so.
</para>
<para>
Although <productname>PostgreSQL</productname> does not attempt to enforce
constraints on foreign tables, it does assume that they are correct
for purposes of query optimization. If there are rows visible in the
foreign table that do not satisfy a declared constraint, queries on
the table might produce errors or incorrect answers. It is the user's
responsibility to ensure that the constraint definition matches
reality.
</para>
<caution>
<para>
When a foreign table is used as a partition of a partitioned table,
there is an implicit constraint that its contents must satisfy the
partitioning rule. Again, it is the user's responsibility to ensure
that that is true, which is best done by installing a matching
constraint on the remote server.
</para>
</caution>
<para>
Within a partitioned table containing foreign-table partitions,
an <command>UPDATE</command> that changes the partition key value can
cause a row to be moved from a local partition to a foreign-table
partition, provided the foreign data wrapper supports tuple routing.
However, it is not currently possible to move a row from a
foreign-table partition to another partition.
An <command>UPDATE</command> that would require doing that will fail
due to the partitioning constraint, assuming that that is properly
enforced by the remote server.
</para>
<para>
Similar considerations apply to generated columns. Stored generated
columns are computed on insert or update on the local
<productname>PostgreSQL</productname>