public ImmutableBitSet getOuterJoinFactors(int factIdx) {
    return outerJoinFactors[factIdx];
  }
Returns a bitmap containing the factors that a null-generating factor is
dependent upon, if the factor is null-generating in a left or right outer
join; otherwise null is returned.

@param factIdx Factor for which information will be returned
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveLoptMultiJoin.java