如何读懂执行计划一
SQL> select count(*) from dba_objects;COUNT(*)----------11345Execution Plan----------------------Plan hash value: 2598313856-----------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| 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$sql where sql_text like '%select count(*) from dba_objects%';SQL_ID-------------SQL_TEXT--------------------------------------------gxk8zvq0j02z8select sql_id,sql_text from v$sql where sql_text like '%select count(*) from dba_objects%'1jhx4zbub8uw3select * from v$sql where sql_text like '%select count(*) from dba_objects%'8vcrngun00v6gselect sql_id,sql_text from v$sql where sql_text like '%select count(*) from dbaSQL_ID-------------SQL_TEXT--------------------------------------------_objects%'g4pkmrqrgxg3bselect count(*) from dba_objects08bm3s8hxudu7EXPLAIN PLAN SET STATEMENT_ID='PLUS886' FOR select count(*) from dba_objectsSQL>--===============================SQL> select id,parent_id from v$sql_plan where sql_id='g4pkmrqrgxg3b';ID PARENT_ID---------- ----------01 02 13 24 35 46 57 58 49 810 3ID PARENT_ID---------- ----------11 1012 1013 1214 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?
