查看SQL执行计划 explain

explain使用方式

alter session set current_schema=test;
explain plan for +sql语句;   --并不会实际执行，因此生成的执行计划也是预估的
select * from table(dbms_xplan.display);

explain使用场景

1.内存中没有谓词信息了，查看谓词信息。

2.添加hint或者改写SQL后，看预估执行计划。

3.收集完统计信息后，看新的rows评估情况。或者看不同绑定变量的评估情况。

explain查看执行计划示例

create table t1014 as select * from dba_objects; 

SYS@db11g> explain plan for select object_name from t1014 where object_id<100;

Explained.

SYS@db11g> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------
Plan hash value: 2622338484

---------------------------------------------------------------------------
| Id  | Operation         | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |       |    14 |  1106 |   337   (1)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T1014 |    14 |  1106 |   337   (1)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("OBJECT_ID"<100)

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

17 rows selected.