--DISTINCT测试前的准备
drop table t purge;
create table t as select * from dba_objects;
update t set object_id=rownum;
alter table T modify OBJECT_ID not null;
update t set object_id=2;
update t set object_id=3 where rownum<=25000;
commit;








/*
在oracle10g的R2环境之后，DISTINCT由于其 HASH UNIQUE的算法导致其不会产生排序，其调整的
ALTER SESSION SET "_gby_hash_aggregation_enabled" = FALSE
*/
set linesize 1000
set autotrace traceonly


select  distinct object_id from t ;
执行计划
-----------------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      | 88780 |  1127K|       |   717   (1)| 00:00:09 |
|   1 |  HASH UNIQUE       |      | 88780 |  1127K|  1752K|   717   (1)| 00:00:09 |
|   2 |   TABLE ACCESS FULL| T    | 88780 |  1127K|       |   292   (1)| 00:00:04 |
-----------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1047  consistent gets
          0  physical reads
          0  redo size
        462  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          2  rows processed




/*不过虽然没有排序，通过观察TempSpc可知distinct消耗PGA内存进行HASH UNIQUE运算，
接下来看看建了索引后的情况，TempSpc关键字立即消失，COST也立即下降许多，具体如下*/


--为T表的object_id列建索引
create index idx_t_object_id on t(object_id);
set linesize 1000
set autotrace traceonly


select  /*+index(t)*/ distinct object_id from t ;
执行计划
--------------------------------------------------------------------------------------
| Id  | Operation          | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |                 | 88780 |  1127K|   582   (1)| 00:00:07 |
|   1 |  SORT UNIQUE NOSORT|                 | 88780 |  1127K|   582   (1)| 00:00:07 |
|   2 |   INDEX FULL SCAN  | IDX_T_OBJECT_ID | 88780 |  1127K|   158   (1)| 00:00:02 |
--------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        145  consistent gets
          0  physical reads
          0  redo size
        462  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          2  rows processed