如何读懂执行计划一

SQL> select count(*) from dba_objects;
COUNT(*)
----------
11345

Execution Plan
----------------------------------------------------------
Plan hash value: 2598313856
--------------------------------------------------------------------------------
---------------
| Id | Operation | Name | Rows | Bytes | Cost (%C
PU)| Time |
--------------------------------------------------------------------------------
---------------
| 0 | SELECT STATEMENT | | 1 | | 38
(6)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | |
| |
| 2 | VIEW | DBA_OBJECTS | 10010 | | 38
(6)| 00:00:01 |
| 3 | UNION-ALL | | | |
| |
|* 4 | FILTER | | | |
| |
|* 5 | HASH JOIN | | 11645 | 398K| 35
(6)| 00:00:01 |
| 6 | TABLE ACCESS FULL | USER$ | 32 | 96 | 2
(0)| 00:00:01 |
|* 7 | TABLE ACCESS FULL | OBJ$ | 11645 | 363K| 32
(4)| 00:00:01 |
|* 8 | TABLE ACCESS BY INDEX ROWID| IND$ | 1 | 8 | 2
(0)| 00:00:01 |
|* 9 | INDEX UNIQUE SCAN | I_IND1 | 1 | | 1
(0)| 00:00:01 |
| 10 | NESTED LOOPS | | 3 | 18 | 3
(0)| 00:00:01 |
| 11 | INDEX FULL SCAN | I_LINK1 | 3 | 9 | 1
(0)| 00:00:01 |
| 12 | TABLE ACCESS CLUSTER | USER$ | 1 | 3 | 1
(0)| 00:00:01 |
|* 13 | INDEX UNIQUE SCAN | I_USER# | 1 | | 0
(0)| 00:00:01 |
doc的前半句话不能很好的理解，不知道它要表述什么意思?
反正我们只需要看第三列"|"和语句对比，看哪个语句动作缩的
越靠右侧就先执行它，如果存在相同的则谁在前面就执行谁，根据这个
指导原则，计划的执行顺序如下：
6,7,5,9,8,4,11,13,12,10,3,2,1,0
--===============================
SQL> select sql_id,sql_text from v$sql where sql_text like '%select count(*) fro
m dba_objects%';
SQL_ID
-------------
SQL_TEXT
--------------------------------------------------------------------------------
gxk8zvq0j02z8
select sql_id,sql_text from v$sql where sql_text like '%select count(*) from dba
_objects%'
1jhx4zbub8uw3
select * from v$sql where sql_text like '%select count(*) from dba_objects%'
8vcrngun00v6g
select sql_id,sql_text from v$sql where sql_text like '%select count(*) from dba

SQL_ID
-------------
SQL_TEXT
--------------------------------------------------------------------------------
_objects%'
g4pkmrqrgxg3b
select count(*) from dba_objects
08bm3s8hxudu7
EXPLAIN PLAN SET STATEMENT_ID='PLUS886' FOR select count(*) from dba_objects

SQL>
--===============================
SQL> select id,parent_id from v$sql_plan where sql_id='g4pkmrqrgxg3b';
ID PARENT_ID
---------- ----------
0
1 0
2 1
3 2
4 3
5 4
6 5
7 5
8 4
9 8
10 3
ID PARENT_ID
---------- ----------
11 10
12 10
13 12
14 rows selected.
SQL>
9i的执行计划显示出来的是2列数字：
看起来就更容易了：
先从最上面看id，之后赵它的parent_id,如此下去直到找不到
parent_id,那么就先执行这个id，遇到具有相同parent_id的则谁
在前面执行谁，按照这种方法排列的顺序如下
6,7,5,9,8,4,11,13,12,10,3,2,1,0
其实我们可以根据id和parent_id画一颗树，然后按照中序（应该是中序吧）
遍历这颗树即可