请问，下面的这条语句该怎么优化？
Select Count(*) as usercount from (

SELECT UserID FROM IPTV_COOK_PROGRESS where RowTime between '08-08-01' and '08-08-31'

UNION SELECT UserID FROM IPTV_FISH_HISTORY where RowTime between '08-08-01' and '08-08-31'

UNION SELECT UserID FROM IPTV_FIND_HISTORY where RowTime between '08-08-01' and '08-08-31'

UNION SELECT UserID FROM IPTV_QUIZ_HISTORY where RowTime between '08-08-01' and '08-08-31') a

谢谢。我分别在RowTime_UserID，RowTime，UserID上面创建了索引，查询百万条数据，从1万6毫秒提高到2百毫秒以内。