public int [] getJoinFieldRefCounts(int factIdx) {
    return requireNonNull(
            joinFieldRefCountsMap.get(factIdx),
            () -> "no entry in joinFieldRefCountsMap found for " + factIdx);
  }
Returns the join field reference counts for a factor.

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