2 types of selectivity estimation improvements need to be done:
1. For range predicates on the same column, we need to collect all such predicates in 1 group and do a histogram lookup for them together.
Currently, the Drill behavior is to treat each of the conjuncts independently and multiply the individual selectivities. However, that will not give the accurate estimates. Here, we want to group the predicates on 'a' together and do a single lookup. Similarly for 'b'.
2. NULLs are not maintained by the histogram but when doing the selectivity calculations, the histogram should use the totalRowCount as the denominator rather than the non-null count.