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