public @Nullable Integer getJoinRemovalFactor(int dimIdx) {
    return joinRemovalFactors[dimIdx];
  }
Returns the factor id of the fact table corresponding to a dimension
table in a semi-join, in the case where the join with the dimension table
can be removed.

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