Consider number of predicates at each level in rewriting cost model #3681
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This PR improves the rewriting cost model by making it consider the number of predicates in each expression at each level of the expression graph, instead of just looking at the lowest expression with no predicates at all. This is in order for the cost model to prefer queries with the same number of predicates, where some predicates were pushed from higher levels in the QGM to lower levels, which can lead to better index matching in the planning phase.
Example: the cost model should prefer the logical plan
SELECT a FROM (SELECT a, b FROM T WHERE a = 42) WHERE EXISTS (SELECT a FROM T2)overSELECT a FROM (SELECT a, b, d FROM T) WHERE a = 42 AND EXISTS (SELECT a FROM T2).