dblink_cancel_query
Description
  
   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
  
  .
 
Return Value
Returns OK if the cancel request has been sent, or the text of an error message on failure.