Are you sure that you cannot convert that append to something else?
Break up the append into multiple appends with smaller numbers of events
@ITWhisperer the append result has 33k records. how to spilt it and it may chance to increase in future?
It depends on what your subsearch is - can you split by time for example? The main thing you need to split is the initial part of the subsearch. If you can't use time, what about filtering the search on other fields such as host or source.