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
Name of the connection to use.
OK if the cancel request has been sent, or
the text of an error message on failure.