/*
  这也是一个真实的故事，为了提高建索引的效率，采用了并行的方式，并且设到了索引的属性中去了，引发了性能故障。
  一般来说，如果我们要做并行的操作，建议用HINT的方式给查询语句加索引，比如/*+parallel n*/
*/

drop table t purge;
create table t as select * from dba_objects where object_id is not null;
alter table T modify object_id not null;
insert into t  select * from t;
insert into t  select * from t;
insert into t  select * from t;
insert into t  select * from t;
insert into t  select * from t;
insert into t  select * from t;
insert into t  select * from t;
commit;


set timing on
create index idx_object_id on t(object_id) parallel 8;


索引已创建。


已用时间:  00: 00: 09.85




select index_name,degree from user_indexes where table_name='T';
INDEX_NAME                     DEGREE
------------------------------ -------
IDX_OBJECT_ID                  8

set linesize 1000
set autotrace traceonly

select count(*) from t;
执行计划
----------------------------------------------------------------------------------------------------------------
| Id  | Operation                 | Name          | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT          |               |     1 |  5797   (2)| 00:01:10 |        |      |         |
|   1 |  SORT AGGREGATE           |               |     1 |            |          |        |      |         |
|   2 |   PX COORDINATOR          |               |       |            |          |        |      |         |
|   3 |    PX SEND QC (RANDOM)    | :TQ10000      |     1 |            |          |  Q1,00 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE        |               |     1 |            |          |  Q1,00 | PCWP |         |
|   5 |      PX BLOCK ITERATOR    |               |  8100K|  5797   (2)| 00:01:10 |  Q1,00 | PCWC |         |
|   6 |       INDEX FAST FULL SCAN| IDX_OBJECT_ID |  8100K|  5797   (2)| 00:01:10 |  Q1,00 | PCWP |         |
----------------------------------------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
         24  recursive calls
          0  db block gets
      25365  consistent gets
      20769  physical reads
          0  redo size
        426  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
          


set autotrace off
alter index   IDX_OBJECT_ID noparallel;
select index_name,degree from user_indexes where table_name='T';
INDEX_NAME                     DEGREE
------------------------------ -------
IDX_OBJECT_ID                  1        


SQL> select count(*) from t;
执行计划
-------------------------------------------------------------------------------
| Id  | Operation             | Name          | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |               |     1 |  5797   (2)| 00:01:10 |
|   1 |  SORT AGGREGATE       |               |     1 |            |          |
|   2 |   INDEX FAST FULL SCAN| IDX_OBJECT_ID |  8100K|  5797   (2)| 00:01:10 |
-------------------------------------------------------------------------------
统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
      20828  consistent gets
          0  physical reads
          0  redo size
        426  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