环境：Oracle 11.2.0.3
需求：生产一张表由于前期设计不当，没有主键。现需要添加主键，数据量很大，想并行建立。

1.直接添加,提示ora-3001:未实施的功能;只能单线程建立主键

SQL> alter table t add constraint pk_t primary key (object_id) using index online parallel 2;

alter table t add constraint pk_t primary key (object_id) using index online parallel 2

ORA-03001: 未实施的功能

SQL> alter table t add constraint pk_t primary key (object_id) using index online;

Table altered

SQL> alter table t drop primary key;

Table altered

2.考虑先并行添加惟一性索引，再添加主键

SQL> create unique index pk_t on t(object_id) parallel 2 online;

Index created

SQL> alter table t add constraint pk_t primary key (object_id);

Table altered

SQL> alter index pk_t noparallel;

Index altered

3.对比主键和惟一性索引的区别

SQL> desc t
Name           Type          Nullable Default Comments
-------------- ------------- -------- ------- --------
OWNER          VARCHAR2(30)  Y                       
OBJECT_NAME    VARCHAR2(128) Y                       
SUBOBJECT_NAME VARCHAR2(30)  Y                       
OBJECT_ID      NUMBER                                
DATA_OBJECT_ID NUMBER        Y                       
OBJECT_TYPE    VARCHAR2(19)  Y                       
CREATED        DATE          Y                       
LAST_DDL_TIME  DATE          Y                       
TIMESTAMP      VARCHAR2(19)  Y                       
STATUS         VARCHAR2(7)   Y                       
TEMPORARY      VARCHAR2(1)   Y                       
GENERATED      VARCHAR2(1)   Y                       
SECONDARY      VARCHAR2(1)   Y                       
NAMESPACE      NUMBER        Y                       
EDITION_NAME   VARCHAR2(30)  Y                       

SQL> alter table t drop primary key;

Table altered

SQL> desc t
Name           Type          Nullable Default Comments
-------------- ------------- -------- ------- --------
OWNER          VARCHAR2(30)  Y                       
OBJECT_NAME    VARCHAR2(128) Y                       
SUBOBJECT_NAME VARCHAR2(30)  Y                       
OBJECT_ID      NUMBER        Y                       
DATA_OBJECT_ID NUMBER        Y                       
OBJECT_TYPE    VARCHAR2(19)  Y                       
CREATED        DATE          Y                       
LAST_DDL_TIME  DATE          Y                       
TIMESTAMP      VARCHAR2(19)  Y                       
STATUS         VARCHAR2(7)   Y                       
TEMPORARY      VARCHAR2(1)   Y                       
GENERATED      VARCHAR2(1)   Y                       
SECONDARY      VARCHAR2(1)   Y                       
NAMESPACE      NUMBER        Y                       
EDITION_NAME   VARCHAR2(30)  Y                       

总结：
对于已有大量数据的表无法并行建立主键，可以先并行建立惟一性索引然后再加上主键。
主键不能为空，惟一性索引可以为空。