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