--MAX/MIN 的索引優(yōu)化

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drop table t purge;

create table t as select * from dba_objects;

update t set object_id=rownum;

alter table t add constraint pk_object_id primary key (OBJECT_ID);

set autotrace on

set linesize 1000

select max(object_id) from t;

執(zhí)行計劃

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

| Id  | Operation                  | Name         | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT           |              |     1 |    13 |     2   (0)| 00:00:01 |

|   1 |  SORT AGGREGATE            |              |     1 |    13 |            |          |

|   2 |   INDEX FULL SCAN (MIN/MAX)| PK_OBJECT_ID |     1 |    13 |     2   (0)| 00:00:01 |

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

統(tǒng)計信息

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

          0  recursive calls

          0  db block gets

          2  consistent gets

          0  physical reads

          0  redo size

        431  bytes sent via SQL*Net to client

        415  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

--最小值老師的試驗就無需展現(xiàn)執(zhí)行計劃結(jié)果了，必然和最大值的執(zhí)行計劃一樣！          

select min(object_id) from t;

--如果沒用到索引的情況是如下,請看看執(zhí)行計劃有何不同，請看看代價和邏輯讀的差異！

select /*+full(t)*/ max(object_id) from t;

執(zhí)行計劃

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

| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT   |      |     1 |    13 |   292   (1)| 00:00:04 |

|   1 |  SORT AGGREGATE    |      |     1 |    13 |            |          |

|   2 |   TABLE ACCESS FULL| T    | 92407 |  1173K|   292   (1)| 00:00:04 |

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

統(tǒng)計信息

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

          0  recursive calls

          0  db block gets

       1047  consistent gets

          0  physical reads

          0  redo size

        431  bytes sent via SQL*Net to client

        415  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

---另外，可以做如下試驗觀察在有索引的情況下，隨這記錄數(shù)增加，性能差異是否明顯？

set autotrace off

drop table t_max purge;

create table t_max as select * from dba_objects;

insert into t_max select * from t_max;

insert into t_max select * from t_max;

insert into t_max select * from t_max;

insert into t_max select * from t_max;

insert into t_max select * from t_max;

select count(*) from t_max;

create index idx_t_max_obj on t_max(object_id);

set autotrace on 

select max(object_id) from t_max;

執(zhí)行計劃

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

| Id  | Operation                  | Name          | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT           |               |     1 |    13 |     3   (0)| 00:00:01 |

|   1 |  SORT AGGREGATE            |               |     1 |    13 |            |          |

|   2 |   INDEX FULL SCAN (MIN/MAX)| IDX_T_MAX_OBJ |     1 |    13 |     3   (0)| 00:00:01 |

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

統(tǒng)計信息

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

          0  recursive calls

          0  db block gets

          3  consistent gets

          0  physical reads

          0  redo size

        431  bytes sent via SQL*Net to client

        415  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

          1  rows processed

/*

object_id如果允許為空，加個索引后，會走INDEX FULL SCAN (MIN/MAX)高效算法嗎，

當(dāng)然會了！取最大最小還怕啥空值？

*/ 

drop table t purge;

create table t as select * from dba_objects ;

create index idx_object_id on t(object_id);

set autotrace on

set linesize 1000

select max(object_id) from t;