SQL> set autotrace
Usage: SET AUTOT[RACE] {OFF | ON | TRACE[ONLY]} [EXP[LAIN]] [STAT[ISTICS]]

set autot on
--打开autotrace，之后执行的sql，会显示sql执行结果、执行计划、统计信息

set autot on exp
--会显示sql执行结果、执行计划

set autot on stat
--会显示sql执行结果、统计信息

set autot trace
--只显示执行计划、统计信息

set autot trace exp
--只显示执行计划（可能不准,sql查询并没有真正执行）

set autot trace stat
--只显示统计信息

set autot off
--关闭autotrace

SQL> set autot trace exp
SQL> select * from t_jingyu;
Elapsed: 00:00:00.04

Execution Plan
----------------------------------------------------------
Plan hash value: 2809386205

------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |  1864K|    69M|   981   (2)| 00:00:12 |
|   1 |  TABLE ACCESS FULL| T_JINGYU |  1864K|    69M|   981   (2)| 00:00:12 |
------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL> set autot trace    
SQL> select * from t_jingyu;

2097152 rows selected.

Elapsed: 00:00:24.89

Execution Plan
----------------------------------------------------------
Plan hash value: 2809386205

------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |  1864K|    69M|   981   (2)| 00:00:12 |
|   1 |  TABLE ACCESS FULL| T_JINGYU |  1864K|    69M|   981   (2)| 00:00:12 |
------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
     143066  consistent gets
       3484  physical reads
          0  redo size
   51171186  bytes sent via SQL*Net to client
    1538429  bytes received via SQL*Net from client
     139812  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
    2097152  rows processed

SQL> --试验表明set autot trace exp不真正执行sql显示的执行计划，set autot trace 执行了sql显示的执行计划。