hive同一張表union all的優化

insert overwrite table dwd.dwd_user_play_event_1d_delta_daily partitions (dt = '${DT}') 
select *
  from ods.user_event_log_ex 
 where dt = '${DT}' and event_name = 'play_start'
 union all 
select *
  from ods.user_event_log_ex 
 where dt = '${DT}' and event_name = 'play_end' and play_time > 0
 union all
select *
  from ods.user_event_log_ex
 where dt = '${DT}' and event_name = 'play_error' and error_code is not null

從上SQL中可以看出都是從同一張表中拉取數據，只是條件不一樣，但上面的SQL需要對同一張表掃描三次，每次union all都會產生mapreduce，都是需要消耗資源和時間的，可對以上做如下優化，減少對錶的掃描次數和mapreduce的產生。

from ods.user_event_log_ex 
inser overwrite table dwd.dwd_user_play_event_1d_delta_daily partition (dt = '${DT}')
select *
  where dt ='${DT}' and event_name = 'play_start'
insert overwrite table dwd.dwd_user_play_event_1d_delta_daily partition (dt = '${DT}')
select *
  where dt ='${DT}' and event_name = 'play_end' and play_time > 0
insert overwrite table dwd.dwd_user_play_event_1d_delta_daily partition (dt = '${DT}')
select *
  where dt ='${DT}' and event_name = 'play_error' and error_code is not null