liyafan82 opened a new pull request #2330:
URL: https://github.com/apache/calcite/pull/2330
According to our current implementation (`RelMdDistinctRowCount`),
estimating the number of distinctive values (NDV) does not make good use of the
filter condition. It simply forwards the call to its input operator with the
fiter condition attached.
In fact, more information can be obtained for some special but commonly used
conditions. For example, given condition `x = 'a'`, we can deduce that `NDV( x
) <= 1`. Given condition `x in ('a', 'b')`, we can deduce that `NDV( x ) <= 2`.
More generally, if we have `x in ('a', 'b') AND y in ('c', 'd', 'e')`, we
have `NDV(x, y) <= 2 * 3 = 6`.
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