Drop超大表

1.清空数据并且保留原来的extent
truncate table table_name reuse storage;

2.逐步回缩extent（不能超过2000000M）.
ALTER table table_name DEALLOCATE UNUSED KEEP 175000M;
ALTER table table_name DEALLOCATE UNUSED KEEP 172000M;
ALTER table table_name DEALLOCATE UNUSED KEEP 169000M;
ALTER table table_name DEALLOCATE UNUSED KEEP 166000M;

3.drop table subscription_history_old ;