Github user chenghao-intel commented on the pull request:
https://github.com/apache/spark/pull/482#issuecomment-41232857
@marmbrus I will review the nullability for aggregate operators etc. and
it also may could be folded by statically analyze the nullability. (for example
"sum(a)" but "a" could be an expression "Always Be Null").
I was thinking not to extend the original nullability of expression,
however only the "Never Be Null" and "Always Be Null" will be helpful in
constant folding rule, but the boolean value of nullability is only represented
as "Never Be Null" or "Possibly Be Null".
I think we need to find some other way to show "Always Be Null" state for
expression if we don't want to extend the nullability.
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