SORT ORDER BY STOPKEY

select * from 

(

select * from 

(

select  a.*,rownum rn

  from page a 

 where object_id >1000 and owner='SYS'

 order by object_id desc

) where rownum<=20

) where rn>=20;







现在加个HINT



SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));



PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------------------------------------------------------------

SQL_ID  9w6p4hu1q1z4n, child number 0

-------------------------------------

select * from ( select * from ( select /*+ index(a) */ a.*,rownum rn   from page a  where object_id >1000 and owner='SYS'

order by object_id desc ) where rownum<=20 ) where rn>=0



Plan hash value: 3486388599



------------------------------------------------------------------------------------------------------------------------------------

| Id  | Operation                        | Name       | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |

------------------------------------------------------------------------------------------------------------------------------------

|*  1 |  VIEW                            |            |      1 |     20 |     20 |00:00:01.28 |     739 |       |       |          |

|*  2 |   COUNT STOPKEY                  |            |      1 |        |     20 |00:00:01.28 |     739 |       |       |          |

|   3 |    VIEW                          |            |      1 |  22595 |     20 |00:00:01.28 |     739 |       |       |          |

|*  4 |     SORT ORDER BY STOPKEY        |            |      1 |  22595 |     20 |00:00:01.28 |     739 |  1234K|   574K| 1096K (0)|

|   5 |      COUNT                       |            |      1 |        |  22130 |00:00:01.22 |     739 |       |       |          |

|   6 |       TABLE ACCESS BY INDEX ROWID| PAGE       |      1 |  22595 |  22130 |00:00:01.22 |     739 |       |       |          |

|*  7 |        INDEX RANGE SCAN          | IDX_PAGE_3 |      1 |  22595 |  22130 |00:00:01.20 |     150 |       |       |          |

------------------------------------------------------------------------------------------------------------------------------------



Predicate Information (identified by operation id):

---------------------------------------------------



   1 - filter("RN">=0)

   2 - filter(ROWNUM<=20)

   4 - filter(ROWNUM<=20)

   7 - access("OBJECT_ID">1000 AND "OWNER"='SYS' AND "OBJECT_ID" IS NOT NULL)

       filter("OWNER"='SYS')







|*  4 |     SORT ORDER BY STOPKEY        |            |      1 |  22595 |     20 |







SORT ORDER BY STOPKEY 是指:



排序过后取数据 



走索引后返回 100w条 



是不是要对100w排序