DBMS_XPLAN是Oracle提供的一個用於查看SQL計劃，包括執行計劃和解釋計劃的包；在以前查看SQL執行計劃的時候，我都是使用set autotrace命令，不過現在看來，DBMS_XPLAN包給出了更加簡化的獲取和顯示計劃的方式。
這5個函數分別對應不同的顯示計劃的方式，DBMS_XPLAN包不僅可以獲取解釋計劃，它還可以用來輸出存儲在AWR，SQL調試集，緩存的SQL游標，以及SQL基線中的語句計劃，實現如上的功能，通常會用到一下5個方法：
1.DISPLAY
2.DISPLAY_AWR
3.DISPLAY_CURSOR
4.DISPLAY_PLAN
5.DISPLAY_SQL_PLAN_BASELINE
6.DISPLAY_SQLSET
下面將重點討論關於DBMS_XPLAN包在解釋計劃和執行計劃上的應用。
來看一個經常使用的查看某條語句的解釋計劃示例：
復制代碼 代碼如下:
SQL> explain plan for select * from scott.emp;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
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 |
--------------------------------------------------------------------------
8 rows selected.
-- 對應autotrace實現
SQL> set autotrace traceonly explain
上例中使用了dbms_xplan.display方法來顯示PLAN_TABLE中保存的解釋計劃，如果想要顯示執行計劃，就需要使用到DMBS_XPLAN.DISPLAY_CURSOR方法了，DMBS_XPLAN.DISPLAY_CURSOR調用簽名：
復制代碼 代碼如下:
DBMS_XPLAN.DISPLAY_CURSOR(
sql_id IN VARCHAR2 DEFAULT NULL,
child_number IN NUMBER DEFAULT NULL,
format IN VARCHAR2 DEFAULT 'TYPICAL');
sql_id表示存儲在cursor cache中的SQL語句的id，child_number用於指示緩存sql語句計劃的子id，format參數用於控制包含在輸出中的信息類型，官檔的參數如下：
1.BASIC: 顯示最少的信息，只包括操作類型，ID名稱和選項。
2.TYPICAL: 默認值，顯示相關信息以及某些附加的顯示選項，如分區和並發使用等。
3.SERIAL: 與TYPICAL類型相似，區別是它不包括並發的信息，即使是並行執行的計劃。
4.ALL: 顯示最多的信息，包含了TYPICAL的全部以及更多的附加信息,如別名和遠程調用等。
除了以上的基本的四種輸出格式外，format還有一些附加的選項可用於定制化輸出行為，使用中可以通過逗號和空格分隔來聲明多個關鍵字，同時可以使用”+”和”-”符號來包含或排除相應的顯示元素，這些附加的選項在官檔中也有記載：
1.ROWS – 顯示被優化器估算的記錄的行號
2.BYTES – 顯示優化器估算的字節數
3.COST – 顯示優化器計算的成本信息
4.PARTITION – 顯示分區的分割信息
5.PARALLEL – 顯示並行執行信息
6.PREDICATE – 顯示謂語
7.PROJECTION – 顯示列投影部分(每一行的那些列被傳遞給其父列已經這些列的大小)
8.ALIAS – 顯示查詢塊名稱已經對象別名
9.REMOTE – 顯示分布式查詢信息
10.NOTE – 顯示注釋
11.IOSTATS – 顯示游標執行的IO統計信息
12.MEMSTATS – 為內存密集運算如散列聯結，排序，或一些類型的位圖運算顯示內存管理統計信息
13.ALLSTATS – 與'IOSTATS MEMSTATS'等價
14.LAST – 顯示最後執行的執行計劃統計信息，默認顯示為ALL類型，並且可以累積。
以上的參數同樣適用於解釋計劃的display方法。
示例部分：
一、使用display_cursor方法查看最近一條語句的執行計劃
復制代碼 代碼如下:
SQL> select /*+ gather_plan_statistics */ count(*) from scott.emp;
COUNT(*)
----------
14
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------
SQL_ID f9qyz8s3c2c02, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ count(*) from scott.emp
Plan hash value: 2937609675
-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 1 |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 1 |
| 2 | INDEX FULL SCAN| PK_EMP | 1 | 14 | 14 |00:00:00.01 | 1 |
-------------------------------------------------------------------------------------
14 rows selected.
使用dbms_xplan.display_cursor(null,null,'ALLSTATS LAST')時，將sql_id和child_number設置成null，表示獲取上一條執行語句的執行計劃；注意上面的例子中一定要指定gather_plan_statistics提示或者手動設置數據庫STATISTICS_LEVEL參數為ALL來使得其抓取行數據源的執行統計信息，這些信息包括行數，一直讀取次數，物理讀次數，物理寫次數以及運算在一行數據上耗費的運行時間，如果沒有指定該提示，就不會有A-Rows,A-Time,Buffers這三列信息。
二、獲取某條指定語句的執行計劃
復制代碼 代碼如下:
SQL> select /*+ gather_plan_statistics */ count(*) from hr.employees where department_id in (90, 100);
COUNT(*)
----------
9
-- 通過v$sql視圖查詢到sql語句的SQL_ID和CHILD_NUMBER
SQL> select sql_id,child_number,sql_text from v$sql
2 where sql_text like '%select /*+ gather_plan_statistics */ count(*)%';
SQL_ID CHILD_NUMBER SQL_TEXT
------------- ------------ ----------------------------------------------------------------------------------------------------
5qxmkvh40yw0p 0 select /*+ gather_plan_statistics */ count(*) from hr.employees where department_id in (90, 100)
bqjrnskvpv51n 0 select sql_id,child_number,sql_text from v$sql where sql_text like '%select /*+ gather_plan_statisti
cs */ count(*)%'
-- 獲取對應的執行計劃
SQL> select * from table(dbms_xplan.display_cursor('5qxmkvh40yw0p',0,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID 5qxmkvh40yw0p, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ count(*) from hr.employees where
department_id in (90, 100)
Plan hash value: 4167091351
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 2 |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 2 |
| 2 | INLIST ITERATOR | | 1 | | 9 |00:00:00.01 | 2 |
|* 3 | INDEX RANGE SCAN| EMP_DEPARTMENT_IX | 2 | 9 | 9 |00:00:00.01 | 2 |
--------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access(("DEPARTMENT_ID"=90 OR "DEPARTMENT_ID"=100))
21 rows selected.
三、通過format參數定制執行計劃輸出信息
復制代碼 代碼如下:
-- 使用ALL來顯示解釋計劃的全部信息
SQL> explain plan for
2 select * from emp e, dept d
3 where e.deptno = d.deptno
4 and e.ename = 'JONES' ;
Explained.
SQL> select * from table(dbms_xplan.display(format=>'ALL'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
Plan hash value: 3625962092
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 58 | 4 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | | | | |
| 2 | NESTED LOOPS | | 1 | 58 | 4 (0)| 00:00:01 |
|* 3 | TABLE ACCESS FULL | EMP | 1 | 38 | 3 (0)| 00:00:01 |
|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 20 | 1 (0)| 00:00:01 |
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
3 - SEL$1 / E@SEL$1
4 - SEL$1 / D@SEL$1
5 - SEL$1 / D@SEL$1
Predicate Information (identified by operation id):
3 - filter("E"."ENAME"='JONES')
4 - access("E"."DEPTNO"="D"."DEPTNO")
Column Projection Information (identified by operation id):
-----------------------------------------------------------
1 - (#keys=0) "E"."EMPNO"[NUMBER,22], "E"."ENAME"[VARCHAR2,10],
"E"."JOB"[VARCHAR2,9], "E"."MGR"[NUMBER,22], "E"."HIREDATE"[DATE,7],
"E"."SAL"[NUMBER,22], "E"."COMM"[NUMBER,22], "E"."DEPTNO"[NUMBER,22],
"D"."DEPTNO"[NUMBER,22], "D"."DNAME"[VARCHAR2,14], "D"."LOC"[VARCHAR2,13]
2 - (#keys=0) "E"."EMPNO"[NUMBER,22], "E"."ENAME"[VARCHAR2,10],
"E"."JOB"[VARCHAR2,9], "E"."MGR"[NUMBER,22], "E"."HIREDATE"[DATE,7],
"E"."SAL"[NUMBER,22], "E"."COMM"[NUMBER,22], "E"."DEPTNO"[NUMBER,22],
"D".ROWID[ROWID,10], "D"."DEPTNO"[NUMBER,22]
3 - "E"."EMPNO"[NUMBER,22], "E"."ENAME"[VARCHAR2,10], "E"."JOB"[VARCHAR2,9],
"E"."MGR"[NUMBER,22], "E"."HIREDATE"[DATE,7], "E"."SAL"[NUMBER,22],
"E"."COMM"[NUMBER,22], "E"."DEPTNO"[NUMBER,22]
4 - "D".ROWID[ROWID,10], "D"."DEPTNO"[NUMBER,22]
5 - "D"."DNAME"[VARCHAR2,14], "D"."LOC"[VARCHAR2,13]
43 rows selected.
-- 去除執行計劃上的字節數和成本統計信息
SQL> select empno, ename from emp e, dept d
2 where e.deptno = d.deptno
3 and e.ename = 'JONES' ;
EMPNO ENAME
---------- ----------
7566 JONES
SQL> select * from table(dbms_xplan.display_cursor(null,null,format=>'ALLSTATS LAST -COST -BYTES'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID 3mypf7d6npa97, child number 1
-------------------------------------
select empno, ename from emp e, dept d where e.deptno = d.deptno and
e.ename = 'JONES'
Plan hash value: 3956160932
------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 8 |
|* 1 | TABLE ACCESS FULL| EMP | 1 | 1 | 1 |00:00:00.01 | 8 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(("E"."ENAME"='JONES' AND "E"."DEPTNO" IS NOT NULL))
19 rows selected.
-- 另一種選項，窺視綁定變量的值，非常方便！！
SQL> variable v_empno number
SQL> exec :v_empno := 7566 ;
PL/SQL procedure successfully completed.
SQL> select * from emp where empno = :v_empno ;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7566 JONES MANAGER 7839 02-APR-81 2975 20
SQL> select * from table(dbms_xplan.display_cursor(null,null,format=>'+PEEKED_BINDS'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID 9q17w9umt58m7, child number 0
-------------------------------------
select * from emp where empno = :v_empno
Plan hash value: 2949544139
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 0 (0)| |
--------------------------------------------------------------------------------------
Peeked Binds (identified by position):
--------------------------------------
1 - :V_EMPNO (NUMBER): 7566
Predicate Information (identified by operation id):
---------------------------------------------------
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
2 - access("EMPNO"=:V_EMPNO)
24 rows selected.
-- 並行查詢信息篩選
SQL> select /*+ parallel(d, 4) parallel (e, 4) */
2 d.dname, avg(e.sal), max(e.sal)
3 from dept d, emp e
4 where d.deptno = e.deptno
5 group by d.dname
6 order by max(e.sal), avg(e.sal) desc;
DNAME AVG(E.SAL) MAX(E.SAL)
-------------- ---------- ----------
SALES 1566.66667 2850
RESEARCH 2175 3000
ACCOUNTING 2916.66667 5000
SQL> select * from table(dbms_xplan.display_cursor(null,null,'TYPICAL -BYTES -COST'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID gahr597f78j0d, child number 0
-------------------------------------
select /*+ parallel(d, 4) parallel (e, 4) */ d.dname, avg(e.sal),
max(e.sal) from dept d, emp e where d.deptno = e.deptno group by
d.dname order by max(e.sal), avg(e.sal) desc
Plan hash value: 3078011448
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Time | TQ |IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | | |
| 1 | PX COORDINATOR | | | | | | |
| 2 | PX SEND QC (ORDER) | :TQ10004 | 4 | 00:00:01 | Q1,04 | P->S | QC (ORDER) |
| 3 | SORT ORDER BY | | 4 | 00:00:01 | Q1,04 | PCWP | |
| 4 | PX RECEIVE | | 4 | 00:00:01 | Q1,04 | PCWP | |
| 5 | PX SEND RANGE | :TQ10003 | 4 | 00:00:01 | Q1,03 | P->P | RANGE |
| 6 | HASH GROUP BY | | 4 | 00:00:01 | Q1,03 | PCWP | |
| 7 | PX RECEIVE | | 14 | 00:00:01 | Q1,03 | PCWP | |
| 8 | PX SEND HASH | :TQ10002 | 14 | 00:00:01 | Q1,02 | P->P | HASH |
|* 9 | HASH JOIN BUFFERED | | 14 | 00:00:01 | Q1,02 | PCWP | |
| 10 | PX RECEIVE | | 4 | 00:00:01 | Q1,02 | PCWP | |
| 11 | PX SEND HASH | :TQ10000 | 4 | 00:00:01 | Q1,00 | P->P | HASH |
| 12 | PX BLOCK ITERATOR | | 4 | 00:00:01 | Q1,00 | PCWC | |
|* 13 | TABLE ACCESS FULL| DEPT | 4 | 00:00:01 | Q1,00 | PCWP | |
| 14 | PX RECEIVE | | 14 | 00:00:01 | Q1,02 | PCWP | |
| 15 | PX SEND HASH | :TQ10001 | 14 | 00:00:01 | Q1,01 | P->P | HASH |
| 16 | PX BLOCK ITERATOR | | 14 | 00:00:01 | Q1,01 | PCWC | |
|* 17 | TABLE ACCESS FULL| EMP | 14 | 00:00:01 | Q1,01 | PCWP | |
--------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
9 - access("D"."DEPTNO"="E"."DEPTNO")
13 - access(:Z>=:Z AND :Z<=:Z) 17 - access(:Z>=:Z AND :Z<=:Z)
38 rows selected.
