请各位帮忙优化一下这个查询语句
请各位帮忙优化一下这个代码，当BandingUnits为空时需要执行两遍查询，数据量多时会很慢，不知如何改进呀
select 
isnull((Select  
  BandingUnits  
  From  
  dbo.MstProductUnits pu  
  Where  
  pu.ProductCode = fw.ProductCode  
  and pu.WarehouseCode = fw.WarehouseCode  
   
  ),(Select  
  BandingUnits  
  From  
  dbo.MstProductUnits pu  
  Where  
  pu.ProductCode = fw.ProductCode  
  and pu.WarehouseCode = 0  
  )) as BandingUnits
from dbo.WrkForwarding as fw


------解决方案--------------------
先分表，建立临时表，分块查找，范围缩小