一、描述
在查看执行计划测试的过程中遇到使用dbms_xplan.display无法查看执行计划，发现是因为之前在测试的过程中打开了set autot on，关闭后(set autot off) ，执行计划可以正常显示。

二、操作过程

SQL> 
SQL> explain plan for select count(*) from tt;

Explained.

Elapsed: 00:00:00.00
SQL> set lines 200 pages 2000
SQL> select * from table(dbms_xplan.display());

9 rows selected.

Elapsed: 00:00:00.02

Execution Plan
----------------------------------------------------------
Plan hash value: 2137789089

---------------------------------------------------------------------------------------------
| Id  | Operation                         | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                  |         |  8168 | 16336 |    29   (0)| 00:00:01 |
|   1 |  COLLECTION ITERATOR PICKLER FETCH| DISPLAY |  8168 | 16336 |    29   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
         14  recursive calls
         12  db block gets
         55  consistent gets
          0  physical reads
          0  redo size
       1124  bytes sent via SQL*Net to client
        519  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          9  rows processed

##上面无法显示执行计划，很奇怪！！！                 
##关闭autot ，再次查看执行计划OK！！！
SQL> set autot off
SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3133740314

-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 | 21556   (1)| 00:04:19 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| TT   |  2762K| 21556   (1)| 00:04:19 |
-------------------------------------------------------------------

9 rows selected.

Elapsed: 00:00:00.02

##解释一下执行计划，首先是做了全表扫描这个很容易理解，sort aggregage 是因为做了count(*) 聚合。
SQL>