在ORACLE資料庫裡通常可以使用如下的四種方法來得到目标SQL的執行計劃:
1,EXPLAIN PLAN指令
2,DBMS_XPLAN包
3,SQLPLUS中的AUTOTRACE開關
4,10046事件
除了第四種方法外,其他的三種方法得到的執行計劃都有可能不準确。在ORACLE資料庫中判斷得到的執行計劃是否準确,就是看目标SQL是否被真正的執行,真正執行過的SQL所對應的執行計劃就是準确的,反之,則有可能不準确。
對于使用第二種方法(DBMS_XPLAN)而言,針對不同的應用場景,你可以選擇如下四種方式的一種。
a, select * from table(dbms_xplan.display)
例如:
SQL> explain plan for select * from hr.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 | 7276 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| EMPLOYEES | 107 | 7276 | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------
8 rows selected.
b,select * from table(dbms_xplan.display_cursor(null,null,'advanced'));
c,select * from table(dbms_xplan.display_cursor('sql_id/hash_value',child_cursor_number,'advanced'));
d,select * from table(dbms_xplan.display_awr('sql_id'));
Note:執行select * from table(dbms_xplan.display)所得到的執行計劃可能是不準确的,因為它隻是用于檢視使用explain plan指令得到的目标SQL的執行計劃,目标SQL此時還沒有真正執行,是以用它得到的執行計劃可能不正确。使用剩下的三種方式得到的執行計劃都是準确的,因為此時的目标SQL都已經被實際執行過了。
對第三種方法(即使用SQLPLUS中的AUTOTRACE)而言,可以有下面幾種方法來開啟:
set autotrace on;(目标SQL都已經被執行)
set autotrace traceonly;(目标SQL都已經被執行)
set autotrace traceonly explain; (對于查詢目标SQL時,是沒有被實際執行,但是如果目标SQL是DML語句時,這個時候DML是實際上已經被執行了)
由于SET AUTOTRACE指令後顯示的執行計劃實際上是來源于調用EXPLIAN PLAN指令,而用EXPLAIN PLAN指令得到的執行計劃有可能不準确(特别是在使用了綁定變量的情況下),是以使用SET AUTOTRACE指令所顯示的執行計劃也有可能不準确。
看一個如下的例子來驗證下使用explain plan和set autotrace指令後得到的執行計劃并不是目标SQL真實執行計劃:
SQL> show user
USER is "HR"
SQL> create table T1 as select * from dba_objects;
Table created.
SQL> insert into t1 select * from t1;
50319 rows created.
SQL> commit;
Commit complete.
SQL> insert into t1 select * from t1;
100638 rows created.
SQL> commit;
Commit complete.
SQL> select count(*) from t1;
COUNT(*)
----------
201276
在表T1的OBJECT_ID列上建立一個單鍵值的B樹索引IDX_T1
SQL> create index idx_t1 on t1(object_id);
Index created.
對T1表收集一下統計資訊:
SQL> exec dbms_stats.gather_table_stats(ownname=>'HR',tabname=>'T1',estimate_percent=>100,cascade=>true);
PL/SQL procedure successfully completed.
建立2個綁定變量X和Y,X=0,Y=100000
SQL> var x number;
SQL> var y number;
SQL> exec :x :=0
PL/SQL procedure successfully completed.
SQL> exec :y :=100000
PL/SQL procedure successfully completed.
檢視如下語句的執行計劃:
SQL> explain plan for select count(*) from t1 where object_id between :x and :y;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 2351893609
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 5 | | |
|* 2 | FILTER | | | | | |
|* 3 | INDEX RANGE SCAN| IDX_T1 | 503 | 2515 | 3 (0)| 00:00:01 |
-----------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(TO_NUMBER(:X)<=TO_NUMBER(:Y))
3 - access("OBJECT_ID">=TO_NUMBER(:X) AND "OBJECT_ID"<=TO_NUMBER(:Y))
16 rows selected.
從上面可以看出使用EXPLAIN PLAN得到的執行計劃顯示目标SQL走的是對索引IDX_T1的索引範圍掃描。
但是實際情況是否是這樣呢?我們實際執行下上面的語句:
SQL> select count(*) from t1 where object_id between :x and :y;
COUNT(*)
----------
201276
用DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ADVANCED')得到目标SQL的真實執行計劃如下所示:
SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID 9dhu3xk2zu531, child number 0
-------------------------------------
select count(*) from t1 where object_id between :x and :y
Plan hash value: 1410530761
--------------------------------------------------------------------------------
-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
|
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
-
| 0 | SELECT STATEMENT | | | | 106 (100)|
|
| 1 | SORT AGGREGATE | | 1 | 5 | |
|
|* 2 | FILTER | | | | |
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|
|* 3 | INDEX FAST FULL SCAN| IDX_T1| 201K| 982K| 106 (7)| 00:00:02
|
--------------------------------------------------------------------------------
-
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
1 - SEL$1
3 - SEL$1 / [email protected]$1
Outline Data
-------------
Peeked Binds (identified by position):
--------------------------------------
1 - :X (NUMBER): 0
2 - :Y (NUMBER): 100000
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(:X<=:Y)
3 - filter(("OBJECT_ID">=:X AND "OBJECT_ID"<=:Y))
Column Projection Information (identified by operation id):
-----------------------------------------------------------
1 - (#keys=0) COUNT(*)[22]
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
51 rows selected.
從上面的執行計劃可以看出,現在目标SQL實際的執行計劃是走對索引IDX_T1的索引快速全掃描,這才是目标SQL的真實的執行計劃,即剛才用EXLPAIN PLAN指令得到的計劃是不準确的。
我們再來看下,使用SET AUTOTRACE 指令的情況。打開目前SESSION的AUTOTRACE:
SQL> set autotrace traceonly
SQL> select count(*) from t1 where object_id between :x and :y;
Execution Plan
----------------------------------------------------------
Plan hash value: 2351893609
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 5 | | |
|* 2 | FILTER | | | | | |
|* 3 | INDEX RANGE SCAN| IDX_T1 | 503 | 2515 | 3 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(TO_NUMBER(:X)<=TO_NUMBER(:Y))
3 - access("OBJECT_ID">=TO_NUMBER(:X) AND "OBJECT_ID"<=TO_NUMBER(:Y))
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
451 consistent gets
0 physical reads
0 redo size
413 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
從上面可以看出使用SET AUTOTRACE TRACEONLY後得到的執行計劃和之前用EXPLAIN PLAN指令得到的執行計劃是一樣的。即此時的SET AUTOTRACE TRACEONLY所得到的執行計劃是不準确的。
結論:通過上面的實驗可以證明使用了SET AUTOTRACE指令後顯示的執行計劃實際上是來源于調用EXPLAIN PLAN指令,而EXPLAIN PLAN指令所得到的執行計劃可能是不準确的(特别是在綁定變量的時候),索引使用SET AUTORACE指令的所顯示的執行計劃可能是不準确的。
ORACLE資料庫還有如下方法得到真實的執行計劃:
如果是ORACLE 10G及其以上的版本,該SQL的執行計劃又已經被ORACLE捕獲并存儲到了REPOSITORY中,在可以使用AWR SQL報告來得到真實的曆史執行計劃。