Why do we need an empty column when doing upsert?

[杨晗]

hi all:
     I'm reading phoenix src code recently, and i found 
PRowImpl.toRowMutations() always adds an empty column named '_0' for non-delete 
upsert. Why?
     I read the comment but i didn't quite understand it. Might someone give me 
an example that illustates why an empty column is ALWAYS necessary?


    Further more, I have to access a phoenix table by both phoenix client and 
hbase API in some cases. If i do not add this empty column explicitly by hbase 
API, it is ok  if i read this row by phoenix? 


Thanks
-William