如果为亿级别的表建主键会非常慢，原因是用建主键的语句是不能用到并行的，需要索引和约束分开建才能快。做一个实验：
drop table test purge;
create table test as select * from dba_objects where object_id is not null;
--按普通的方式建索引，可以看到并行度为1
alter table test add constraint pk_t_object_id primary key (object_id) Nologging parallel 16;select degree from user_indexes s where s.index_name=upper('pk_t_object_id');
1
--按下列的方式两步走，先建唯一性索引，然后加约束
alter table TEST drop constraint PK_T_OBJECT_ID cascade;create unique index ind_t_object_id on test(object_id) Nologging parallel 16;
alter table test add constraint pk_t_object_id primary key (object_id);
select degree from user_indexes s where s.index_name=upper('ind_t_object_id');
16
--最后一定要记住，把索引并行度打回来，不然后果很严重
alter index ind_t_object_id noparallel;
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