public boolean isRemovableOuterJoinFactor(int factIdx) {
    return removableOuterJoinFactors.contains(factIdx);
  }
Return whether the factor corresponds to the null-generating factor in
an outer join that can be removed.

@param factIdx Factor in question
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveLoptMultiJoin.java