dblink_cancel_query — cancels any active query on the named connection
dblink_cancel_query(text connname) returns text
    dblink_cancel_query attempts to cancel any query that
    is in progress on the named connection.  Note that this is not
    certain to succeed (since, for example, the remote query might
    already have finished).  A cancel request simply improves the
    odds that the query will fail soon.  You must still complete the
    normal query protocol, for example by calling
    dblink_get_result.
   
connnameName of the connection to use.
    Returns OK if the cancel request has been sent, or
    the text of an error message on failure.
   
SELECT dblink_cancel_query('dtest1');