[20150709]慎用標量子查詢.txt
[20150709]慎用標量子查詢.txt
--花了幾天的時間除錯sql語句，最終發現是標量子查詢在作怪，原始的語句太複雜，我拿scott做一個例子來說明問題。
1.建立測試環境：
SCOTT@test> @ver1
PORT_STRING VERSION BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.3.0 Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
create view v_deptemp1 as select emp.*,dname,loc from emp,dept where dept.deptno=emp.deptno ;
create view v_deptemp2 as select emp.*,(select dname from dept where dept.deptno=emp.deptno) dname ,(select loc from dept where dept.deptno=emp.deptno )loc from emp ;
--第2種寫法就是標量子查詢。
2.測試1：
create index i_dept_dname on scott.dept (dname);
create index i_emp_deptno on scott.emp (deptno)
SCOTT@test> select * from v_deptemp1 where dname='ACCOUNTING' ;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO DNAME LOC
---------- ---------- --------- ---------- ------------------- ---------- ---------- ---------- -------------- -------------
7782 CLARK MANAGER 7839 1981-06-09 00:00:00 2450 10 ACCOUNTING NEW YORK
7839 KING PRESIDENT 1981-11-17 00:00:00 5000 10 ACCOUNTING NEW YORK
7934 MILLER CLERK 7782 1982-01-23 00:00:00 1300 10 ACCOUNTING NEW YORK
SCOTT@test> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 09tbznuqjnxa4, child number 0
-------------------------------------
select * from v_deptemp1 where dname='ACCOUNTING'
Plan hash value: 2863776355
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 (100)| |
| 1 | NESTED LOOPS | | | | | |
| 2 | NESTED LOOPS | | 5 | 295 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 20 | 2 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | I_DEPT_DNAME | 1 | | 1 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | I_EMP_DEPTNO | 5 | | 0 (0)| |
| 6 | TABLE ACCESS BY INDEX ROWID | EMP | 5 | 195 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$F5BB74E1
3 - SEL$F5BB74E1 / DEPT@SEL$2
4 - SEL$F5BB74E1 / DEPT@SEL$2
5 - SEL$F5BB74E1 / EMP@SEL$2
6 - SEL$F5BB74E1 / EMP@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("DNAME"='ACCOUNTING')
5 - access("DEPT"."DEPTNO"="EMP"."DEPTNO")
--oracle 很好的分析確定索引，並且對emp表也可以使用i_emp_deptno。
--看看第2種情況：
SCOTT@test> select * from v_deptemp2 where dname='ACCOUNTING' ;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO DNAME LOC
---------- ---------- --------- ---------- ------------------- ---------- ---------- ---------- -------------- -------------
7782 CLARK MANAGER 7839 1981-06-09 00:00:00 2450 10 ACCOUNTING NEW YORK
7839 KING PRESIDENT 1981-11-17 00:00:00 5000 10 ACCOUNTING NEW YORK
7934 MILLER CLERK 7782 1982-01-23 00:00:00 1300 10 ACCOUNTING NEW YORK
SCOTT@test> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 45acjyf0f8x54, child number 0
-------------------------------------
select * from v_deptemp2 where dname='ACCOUNTING'
Plan hash value: 2698003519
-------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 13 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| |
| 3 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 11 | 1 (0)| 00:00:01 |
|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| |
|* 5 | VIEW | V_DEPTEMP2 | 14 | 1456 | 3 (0)| 00:00:01 |
| 6 | TABLE ACCESS FULL | EMP | 14 | 546 | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$3 / DEPT@SEL$3
2 - SEL$3 / DEPT@SEL$3
3 - SEL$4 / DEPT@SEL$4
4 - SEL$4 / DEPT@SEL$4
5 - SEL$2 / V_DEPTEMP2@SEL$1
6 - SEL$2 / EMP@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("DEPT"."DEPTNO"=:B1)
4 - access("DEPT"."DEPTNO"=:B1)
5 - filter("DNAME"='ACCOUNTING')
--這樣無論如何都不會使用emp的i_emp_deptno。當然我實際上的情況實際上開發想避免dname 為null的情況。
--因為標量子查詢如果查詢dname為null不是他所需要的。
3.測試2：
--再建立一個表tx
create table tx ( id number,name varchar2(20));
insert into tx values (7654,'MARTIN');
commit;
SCOTT@test> select * from v_deptemp1 v ,tx where tx.id=v.empno and dname is not null;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO DNAME LOC ID NAME
---------- ---------- --------- ---------- ------------------- ---------- ---------- ---------- -------------- ------------- ---------- ------------
7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30 SALES CHICAGO 7654 MARTIN
SCOTT@test> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID gskw0uw49rw0k, child number 0
-------------------------------------
select * from v_deptemp1 v ,tx where tx.id=v.empno and dname is not null
Plan hash value: 2230440165
------------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 5 (100)| |
| 1 | NESTED LOOPS | | | | | |
| 2 | NESTED LOOPS | | 1 | 70 | 5 (0)| 00:00:01 |
| 3 | NESTED LOOPS | | 1 | 50 | 4 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL | TX | 1 | 11 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 39 | 1 (0)| 00:00:01 |
|* 6 | INDEX UNIQUE SCAN | PK_EMP_EMPNO | 1 | | 0 (0)| |
|* 7 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| |
|* 8 | TABLE ACCESS BY INDEX ROWID | DEPT | 1 | 20 | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$F5BB74E1
4 - SEL$F5BB74E1 / TX@SEL$1
5 - SEL$F5BB74E1 / EMP@SEL$2
6 - SEL$F5BB74E1 / EMP@SEL$2
7 - SEL$F5BB74E1 / DEPT@SEL$2
8 - SEL$F5BB74E1 / DEPT@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
6 - access("TX"."ID"="EMP"."EMPNO")
7 - access("DEPT"."DEPTNO"="EMP"."DEPTNO")
8 - filter("DNAME" IS NOT NULL)
--而如果換成v_deptemp2
SCOTT@test> select * from v_deptemp2 v ,tx where tx.id=v.empno and dname is not null;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO DNAME LOC ID NAME
---------- ---------- --------- ---------- ------------------- ---------- ---------- ---------- -------------- ------------- ---------- --------
7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30 SALES CHICAGO 7654 MARTIN
SCOTT@test> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 7b8950at9688x, child number 0
-------------------------------------
select * from v_deptemp2 v ,tx where tx.id=v.empno and dname is not null
Plan hash value: 2887512270
----------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 7 (100)| | | | |
| 1 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 13 | 1 (0)| 00:00:01 | | | |
|* 2 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| | | | |
| 3 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 11 | 1 (0)| 00:00:01 | | | |
|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| | | | |
|* 5 | HASH JOIN | | 1 | 106 | 7 (15)| 00:00:01 | 1156K| 1156K| 383K (0)|
| 6 | TABLE ACCESS FULL | TX | 1 | 11 | 3 (0)| 00:00:01 | | | |
|* 7 | VIEW | V_DEPTEMP2 | 14 | 1330 | 3 (0)| 00:00:01 | | | |
| 8 | TABLE ACCESS FULL | EMP | 14 | 546 | 3 (0)| 00:00:01 | | | |
----------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$3 / DEPT@SEL$3
2 - SEL$3 / DEPT@SEL$3
3 - SEL$4 / DEPT@SEL$4
4 - SEL$4 / DEPT@SEL$4
5 - SEL$1
6 - SEL$1 / TX@SEL$1
7 - SEL$2 / V@SEL$1
8 - SEL$2 / EMP@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("DEPT"."DEPTNO"=:B1)
4 - access("DEPT"."DEPTNO"=:B1)
5 - access("TX"."ID"="V"."EMPNO")
7 - filter("DNAME" IS NOT NULL)
--emp始終選擇的是全表掃描。
4.繼續測試：
--當把dname is not null刪除後，結果呢？
SCOTT@test> select * from v_deptemp2 v ,tx where tx.id=v.empno ;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO DNAME LOC ID NAME
---------- ---------- --------- ---------- ------------------- ---------- ---------- ---------- -------------- ------------- ---------- ---------
7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30 SALES CHICAGO 7654 MARTIN
SCOTT@test> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 1sd5umnwjt823, child number 0
-------------------------------------
select * from v_deptemp2 v ,tx where tx.id=v.empno
Plan hash value: 3698375752
----------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 4 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID | DEPT | 1 | 13 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| |
| 3 | TABLE ACCESS BY INDEX ROWID | DEPT | 1 | 11 | 1 (0)| 00:00:01 |
|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| |
| 5 | NESTED LOOPS | | | | | |
| 6 | NESTED LOOPS | | 1 | 50 | 4 (0)| 00:00:01 |
| 7 | TABLE ACCESS FULL | TX | 1 | 11 | 3 (0)| 00:00:01 |
|* 8 | INDEX UNIQUE SCAN | PK_EMP_EMPNO | 1 | | 0 (0)| |
| 9 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 39 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$3 / DEPT@SEL$3
2 - SEL$3 / DEPT@SEL$3
3 - SEL$4 / DEPT@SEL$4
4 - SEL$4 / DEPT@SEL$4
5 - SEL$F5BB74E1
7 - SEL$F5BB74E1 / TX@SEL$1
8 - SEL$F5BB74E1 / EMP@SEL$2
9 - SEL$F5BB74E1 / EMP@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("DEPT"."DEPTNO"=:B1)
4 - access("DEPT"."DEPTNO"=:B1)
8 - access("TX"."ID"="EMP"."EMPNO")
--可以發現沒有這個條件(dname is not null),emp可以很好的選擇索引。
--實際我們的檢視定義有多處使用標量子查詢，當然我只要把在where出現的改寫成不使用標量子查詢。
--如果不願意修改還有1個簡單的改寫方法，就是：
SCOTT@test> select * from (select * from v_deptemp2 v ,tx where tx.id=v.empno ) where dname is not null ;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO DNAME LOC ID NAME
---------- ---------- --------- ---------- ------------------- ---------- ---------- ---------- -------------- ------------- ---------- --------
7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30 SALES CHICAGO 7654 MARTIN
SCOTT@test> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 8yy4n9t1dqhmm, child number 0
-------------------------------------
select * from (select * from v_deptemp2 v ,tx where tx.id=v.empno )
where dname is not null
Plan hash value: 3563298867
-----------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 4 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID | DEPT | 1 | 13 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| |
| 3 | TABLE ACCESS BY INDEX ROWID | DEPT | 1 | 11 | 1 (0)| 00:00:01 |
|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| |
|* 5 | VIEW | | 1 | 129 | 4 (0)| 00:00:01 |
| 6 | NESTED LOOPS | | | | | |
| 7 | NESTED LOOPS | | 1 | 50 | 4 (0)| 00:00:01 |
| 8 | TABLE ACCESS FULL | TX | 1 | 11 | 3 (0)| 00:00:01 |
|* 9 | INDEX UNIQUE SCAN | PK_EMP_EMPNO | 1 | | 0 (0)| |
| 10 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 39 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$4 / DEPT@SEL$4
2 - SEL$4 / DEPT@SEL$4
3 - SEL$5 / DEPT@SEL$5
4 - SEL$5 / DEPT@SEL$5
5 - SEL$335DD26A / from$_subquery$_001@SEL$1
6 - SEL$335DD26A
8 - SEL$335DD26A / TX@SEL$2
9 - SEL$335DD26A / EMP@SEL$3
10 - SEL$335DD26A / EMP@SEL$3
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("DEPT"."DEPTNO"=:B1)
4 - access("DEPT"."DEPTNO"=:B1)
5 - filter("DNAME" IS NOT NULL)
9 - access("TX"."ID"="EMP"."EMPNO")
--這樣改寫後emp可以使用索引!看來oracle cbo 還是不夠成熟，無法識別這種情況。
5.也許有人問v_deptemp1 與 v_deptemp2是不等價的。繼續測試：
SCOTT@test> create or replace view v_deptemp3 as select emp.*,dname,loc from emp,dept where dept.deptno(+)=emp.deptno ;
View created.
alter table emp modify constraint fk_deptno disable novalidate;
--取消主外來鍵約束。
SCOTT@test> update emp set deptno=90 where empno=7654;
1 row updated.
SCOTT@test> commit ;
Commit complete.
SCOTT@test> select * from v_deptemp3 v ,tx where tx.id=v.empno and dname is not null;
no rows selected
SCOTT@test> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID bwytb3gt4vtx1, child number 0
-------------------------------------
select * from v_deptemp3 v ,tx where tx.id=v.empno and dname is not null
Plan hash value: 2230440165
------------------------------------------------------------------------------------------------
| Id | Operation | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 5 (100)| |
| 1 | NESTED LOOPS | | | | | |
| 2 | NESTED LOOPS | | 1 | 70 | 5 (0)| 00:00:01 |
| 3 | NESTED LOOPS | | 1 | 50 | 4 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL | TX | 1 | 11 | 3 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 39 | 1 (0)| 00:00:01 |
|* 6 | INDEX UNIQUE SCAN | PK_EMP_EMPNO | 1 | | 0 (0)| |
|* 7 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| |
|* 8 | TABLE ACCESS BY INDEX ROWID | DEPT | 1 | 20 | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$EB0D93E2
4 - SEL$EB0D93E2 / TX@SEL$1
5 - SEL$EB0D93E2 / EMP@SEL$2
6 - SEL$EB0D93E2 / EMP@SEL$2
7 - SEL$EB0D93E2 / DEPT@SEL$2
8 - SEL$EB0D93E2 / DEPT@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
6 - access("TX"."ID"="EMP"."EMPNO")
7 - access("DEPT"."DEPTNO"="EMP"."DEPTNO")
8 - filter("DNAME" IS NOT NULL)
--就是這樣定義，oracle 一樣選擇合理的執行方式。
總結：
1.不要大量不顧各種情況的使用標量子查詢，實際上標量子查詢存在許多缺陷，僅僅能查詢1個欄位，僅僅返回1行。
2.並且導致內部巢狀連線，而不能採用hash join等連線方式。假如emp表1萬行，這樣查詢dept表要有1萬次(我上面執行2次標量子查詢，也就是2萬次)。
即使dept有索引，邏輯讀的數量不可小看.
3.另外一個原因我覺得可能是資料結構問題，這種情況如果太遵守正規化，資料結構設計耦合太緊，開發很容易選擇標量子查詢的寫法。
例如：1個表有6個欄位有科室程式碼欄位，這樣要顯示科室名稱，這樣from中寫6次科室表，做連線6次查詢，這樣許多開發更願意選擇標量子查詢寫sql語句。
4.總之要開發合理選擇這種寫法,不能到處濫用。
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/267265/viewspace-1727892/，如需轉載，請註明出處，否則將追究法律責任。
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