 query.  To reduce the risk of misexecution of queries,
   <code class="literal">WHERE</code> clauses are not sent to the remote server unless they use
   only data types, operators, and functions that are built-in or belong to an
   extension that's listed in the foreign server's <code class="literal">extensions</code>
   option.  Operators and functions in such clauses must
   be <code class="literal">IMMUTABLE</code> as well.
   For an <code class="command">UPDATE</code> or <code class="command">DELETE</