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Sometimes it's hard to tell whether one dimension contains a null value or not.
If one dimension had show a null or empty value explicitly, then yes, it contains
null value. But if one dimension's values are all non-null, it still early to say
this dimension does not contain null value. Consider a two row case, first row had
"dimA=1" and "dimB=2", the second row only had "dimA=3". To dimB, its value are "2" and
never showed a null or empty value. But when we combines these two rows, dimB is null
in row 2. So we should iterate all rows to determine whether one dimension contains
a null value.