HINT 不生效？

今天有个徒弟找我看一条SQL

SELECT /*+  use_hash(REGION,MPI) */
    REGION.REGION_CODE,
    REGION.REGION_NAME,
    MPI.YEARS,
    SUM(MPI.MPI_COUNT) SUM_MPI_COUNT
  FROM
    (SELECT /*+ index_ffs(T_REGION T_REGION_INDEX) */ REGION_CODE, REGION_NAME FROM T_REGION 
      where REGION_CODE  is not null
      and REGION_NAME is not null
    ) REGION
  LEFT JOIN
    ( SELECT /*+ index_ffs(MPI_PERSON_INDEX MPI_PERSON_INDEX_INDEX) */ AR_CD,
      TO_CHAR(CREATETIME,"YYYY") YEARS,
      COUNT(1) AS MPI_COUNT
    FROM MPI_PERSON_INDEX
    GROUP BY AR_CD,
      TO_CHAR(CREATETIME,"YYYY")
    ) MPI
  ON REGION.REGION_CODE=MPI.AR_CD
  GROUP BY REGION.REGION_CODE,
    REGION.REGION_NAME,
    MPI.YEARS
SELECT /*+  use_hash(REGION,MPI) */