通过执行计划优化sql语句，查看执行计划3种方式
1）autotrace 指令 备注：该指令仅能在sqlplus窗口执行操作，plsqldevelope程序下无法执行该指令 (本人已验证该操作)
命令说明：
set autotrace off 默认值关闭 autotrace
set autotrace on explain 仅显示执行计划
set autotrace on statistice 显示执行计划统计信息
set autotrace on 该选项包含 explain statistice
sql>set autotrace on
sql>select * from epm;
执行计划：
Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 532 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| EMP | 14 | 532 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
7 consistent gets
0 physical reads
0 redo size
1630 bytes sent via sql*Net to client
523 bytes received via sql*Net from client
2 sql*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
14 rows processed
2）explain plan命令 该指令在plsqldevelope 已经验证
sql> explain plan for select * from employees;
Explained
sql> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1445457117
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 107 | 7383 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| EMPLOYEES | 107 | 7383 | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------
8 rows selected
3）使用Toad,PL/sql Developer工具
工具没验证
以上文档参考文献：http://blog.chinaunix.net/uid-21187846-id-3022916.html
