Oracle set autotrace 时提示:Cannot find the Session Identifier. Check PLUSTRACE role is enabled

SQL> set autotrace
Usage: SET AUTOT[RACE] {OFF | ON | TRACE[ONLY]} [EXP[LAIN]] [STAT[ISTICS]]
SQL> set autotrace on
SP2-0618: Cannot find the Session Identifier.  Check PLUSTRACE role is enabled
SP2-0611: Error enabling STATISTICS report
SQL> conn /as sysdba
Connected.
SQL> drop role plustrace;
drop role plustrace
          *
ERROR at line 1:
ORA-01919: role 'PLUSTRACE' does not exist

SQL> create role plustrace;

Role created.

SQL> grant select on v_$sesstat to plustrace;

Grant succeeded.

SQL> grant select on v_$statname to plustrace;

Grant succeeded.
SQL> grant select on v_$mystat to plustrace;

Grant succeeded.

SQL> grant plustrace to dba with admin option;

Grant succeeded.

SQL> grant plustrace to scott;

Grant succeeded.

SQL> conn scott/tiger
Connected.

SQL> set autotrace on
SQL> select count(*) from emp;

  COUNT(*)
----------
    14

Execution Plan
----------------------------------------------------------
Plan hash value: 2937609675

-------------------------------------------------------------------
| Id  | Operation     | Name   | Rows  | Cost (%CPU)| Time      |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT |      |    1 |    1   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE  |      |    1 |           |      |
|   2 |   INDEX FULL SCAN| PK_EMP |    14 |    1   (0)| 00:00:01 |
-------------------------------------------------------------------

Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      1  consistent gets
      1  physical reads
      0  redo size
    526  bytes sent via SQL*Net to client
    520  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed

SQL>