Why logical column in terms of other logical columns and not physical sourc

Question

Hello
Can someone shed some light that in what scenario ..defining a logical column in terms of other logical column is beneficial compared to defining logical column in terms of physical sources?
I found something on google that said defining in terms of logical columns is like one time thing. I dont understand, even if you define in terms of physical source that' too one time.
In both the case we build an expression specifying which logical or physical columns.
Thank you

Answer 1

well logical columns from physical source, if in logical fact table you can set aggr rules. Logical columns based on other logical columns you can not. Logical columns based on other logical columns will inherit the aggr rules. So for instance, if you create col1 and col2. Both based off physical and both having sum aggr rule. If you create logical column col3 based off these two as such "col1"/"col2" it will produce the following sum(col1)/cum(col2). Now if you really wanted sum(col1/col2) then you should have made col3 based off physical as col1/col2 with aggr rule of sum.
Also of some note, all logical columns based off physical will be performed in the inner most query. So for my example above to get col3 where it is logical col1/logical col2 or sum(col1)/sum(col2) the physical query will be:
select D1.c1/D1.c2
from (
select sum(col1) as c1,
sum(col2) as c2
from tableA
) D1
So my short summary of this, is what or how do you want your sql to be created. Based on this can you lead you to your answer of whether to use logical col based on physical or other logical columns. And unltimately, check the sql generated from your work to ensure the sql queries generated are what you want and expect.
I hope this helps!