IndexProbeNode only has the index descriptor, so the stage builder must assume that the first column of the index key is the value that at runtime must be matched with the join predicate. However, the join could have multiple predicates, and even non-open intervals if a $match predicate is using constant values. The IndexProbeNode should carry this information (in a vector of IETs or in an equivalent structure) in order to be correctly lowered to SBE code