索引系列九--索引特性之有序优化distinct

--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