索引系列九--索引特性之有序优化distinct

--DISTINCT测试前的准备

成都创新互联专注于企业全网营销推广、网站重做改版、麻城网站定制设计、自适应品牌网站建设、H5开发、成都星空体育app最新版本(2024已更新)开发、集团公司官网建设、成都外贸网站建设、高端网站制作、响应式网页设计等建站业务，价格优惠性价比高，为麻城等各大城市提供网站开发制作服务。

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

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

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

| 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

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

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

| 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

名称栏目：索引系列九--索引特性之有序优化distinct
分享地址：/article48/pihgep.html

成都网站建设公司_创新互联，为您提供虚拟主机、服务器托管、用户体验、App设计、网站营销、小程序开发

声明：本网站发布的内容（图片、视频和文字）以用户投稿、用户转载内容为主，如果涉及侵权请尽快告知，我们将会在第一时间删除。文章观点不代表本网站立场，如需处理请联系客服。电话：028-86922220；邮箱：631063699@qq.com。内容未经允许不得转载，或转载时需注明来源：创新互联

猜你还喜欢下面的内容