http://qurtyy.blog.163.com/blog/static/57443681201052112417441/
FORALL 用法小結:
作者:sonic
本文主要翻譯、整理了ORACLE官方文檔上有關FORALL的部份内容,不妥之處,還希望多和大家交流。
在發送語句到SQL引擎前,FORALL語句告知PL/SQL 引擎批挷定輸入集合。盡管FORALL語句包含一個疊代(iteration)模式,它并不一是個FOR循環。其文法為:
FORALL index IN lower_bound..upper_bound sql_statement;
一、如何使用批挷定提高性能(How Do Bulk Binds Improve Performance)
在PL/SQL 和SQL引擎(engines)中,太多的上下文切換(context switches)會影響性能。這個會發生在當一個循環為集合中的每個元素執行一個單個SQL語句時。而使用批挷定能顯著提高性能。下圖顯示PL/SQL引擎 和SQL引擎之間的context switches:(PL/SQL引擎執行存過語句僅發送SQL語句到SQL引擎,SQL引擎執行語句後傳回資料給PL/SQL引擎)
PL/SQL引擎發送一次SQL語句給SQL引擎,在SQL引擎中則為範圍中每個index數字執行一次SQL語句。
PL/SQL挷定操作包含以下三類:
in-bind: When a PL/SQL variable or host variable is stored in the database by an INSERT or UPDATE statement.
out-bind:When a database value is assigned to a PL/SQL variable or a host variable by the RETURNING clause of an INSERT, UPDATE, or DELETE statement.
define: When a database value is assigned to a PL/SQL variable or a host variable by a SELECT or FETCH statement.
在SQL語句中,為PL/SQL變量指定值稱為挷定(binding),
DML語句能傳遞所有集合元素到一個單個操作中,這過程稱為批挷定(bulk binding)。
如果集合有20個元素,批挷定讓你用單個操作等效于執行與20個SELECT,INSERT, UPDATE或DELETE語句。這個技術通過減少在PL/SQL和SQL引擎(engines)間的上下文切換來提高性能。批挷定包括:
1.帶INSERT, UPDATE, and DELETE語句的批挷定:在FORALL語句中嵌入SQL語句
2.帶SELECT語句的批挷定:在SELECT語句中用BULK COLLECT 語句代替INTO
下邊的例子分别用FOR和FORALL進行資料插入,以顯示用批挷定的對性能的提高:
Java代碼
![](https://img.laitimes.com/img/_0nNw4CM6IyYiwiM6ICdiwiIml2ZukHcvN2Xu92Yp9CXzV2Zh1WavwVbvNmLllXZhZXYq5yZuFWdodHavw1LcpDc0RHaiojIsJye.gif)
- SQL> SET SERVEROUTPUT ON
- SQL> CREATE TABLE parts (pnum NUMBER(4), pname CHAR(15));
- Table created.
- SQL> DECLARE
- 2 TYPE NumTab IS TABLE OF parts.pnum%TYPE INDEX BY BINARY_INTEGER;
- 3 TYPE NameTab IS TABLE OF parts.pname%TYPE INDEX BY BINARY_INTEGER;
- 4 pnums NumTab;
- 5 Pnames NameTab;
- 6 t1 NUMBER;
- 7 t2 NUMBER;
- 8 t3 NUMBER;
- 9 BEGIN
- 10 FOR i IN 1..500000 LOOP
- 11 pnums(i) := i;
- 12 pnames(i) := 'Part No.'||to_char(i);
- 13 END LOOP;
- 14 t1 := dbms_utility.get_time;
- 15
- 16 FOR i IN 1..500000 LOOP
- 17 INSERT INTO parts VALUES(pnums(i),pnames(i));
- 18 END LOOP;
- 19 t2 := dbms_utility.get_time;
- 20
- 21 FORALL i IN 1..500000
- 22 INSERT INTO parts VALUES(pnums(i),pnames(i));
- 23 t3 := dbms_utility.get_time;
- 24
- 25 dbms_output.put_line('Execution Time (secs)');
- 26 dbms_output.put_line('---------------------');
- 27 dbms_output.put_line('FOR loop: ' || TO_CHAR(t2 - t1));
- 28 dbms_output.put_line('FORALL: ' || TO_CHAR(t3 - t2));
- 29 END;
SQL> SET SERVEROUTPUT ON
SQL> CREATE TABLE parts (pnum NUMBER(4), pname CHAR(15));
Table created.
SQL> DECLARE
2 TYPE NumTab IS TABLE OF parts.pnum%TYPE INDEX BY BINARY_INTEGER;
3 TYPE NameTab IS TABLE OF parts.pname%TYPE INDEX BY BINARY_INTEGER;
4 pnums NumTab;
5 Pnames NameTab;
6 t1 NUMBER;
7 t2 NUMBER;
8 t3 NUMBER;
9 BEGIN
10 FOR i IN 1..500000 LOOP
11 pnums(i) := i;
12 pnames(i) := 'Part No.'||to_char(i);
13 END LOOP;
14 t1 := dbms_utility.get_time;
15
16 FOR i IN 1..500000 LOOP
17 INSERT INTO parts VALUES(pnums(i),pnames(i));
18 END LOOP;
19 t2 := dbms_utility.get_time;
20
21 FORALL i IN 1..500000
22 INSERT INTO parts VALUES(pnums(i),pnames(i));
23 t3 := dbms_utility.get_time;
24
25 dbms_output.put_line('Execution Time (secs)');
26 dbms_output.put_line('---------------------');
27 dbms_output.put_line('FOR loop: ' || TO_CHAR(t2 - t1));
28 dbms_output.put_line('FORALL: ' || TO_CHAR(t3 - t2));
29 END;
SQL> /
Execution Time (secs)
---------------------
FOR loop: 2592
FORALL: 358
PL/SQL procedure successfully completed
進而可以看出FORALL語句在性能上有顯著提高。
注釋:SQL語句能涉及多個集合,然而,性能提高隻适用于下标集合(subscripted collections)
二、FORALL 如何影響復原(How FORALL Affects Rollbacks)
在FORALL語句中,如果任何SQL語句執行産生未處理的異常(exception),先前執行的所有資料庫改變都會被復原。然而,如果産生的異常被捕獲并處理,則復原改變到一個隐式的儲存點,該儲存點在每個SQL語句執行前被标記。之前的改變不會被復原。例如:
Java代碼
![](https://img.laitimes.com/img/_0nNw4CM6IyYiwiM6ICdiwiIml2ZukHcvN2Xu92Yp9CXzV2Zh1WavwVbvNmLllXZhZXYq5yZuFWdodHavw1LcpDc0RHaiojIsJye.gif)
- CREATE TABLE emp2 (deptno NUMBER(2), job VARCHAR2(15));
- INSERT INTO emp2 VALUES(10, 'Clerk');
- INSERT INTO emp2 VALUES(10, 'Clerk');
- INSERT INTO emp2 VALUES(20, 'Bookkeeper'); -- 10-char job title
- INSERT INTO emp2 VALUES(30, 'Analyst');
- INSERT INTO emp2 VALUES(30, 'Analyst');
- Comit;
- DECLARE
- TYPE NumList IS TABLE OF NUMBER;
- depts NumList := NumList(10, 20, 30);
- BEGIN
- FORALL j IN depts.FIRST..depts.LAST
- UPDATE emp2 SET job = job || ' (temp)'
- WHERE deptno = depts(j);
- -- raises a "value too large" exception
- EXCEPTION
- WHEN OTHERS THEN
- COMMIT;
- END;
- /
- PL/SQL procedure successfully completed
- SQL> select * from emp2;
CREATE TABLE emp2 (deptno NUMBER(2), job VARCHAR2(15));
INSERT INTO emp2 VALUES(10, 'Clerk');
INSERT INTO emp2 VALUES(10, 'Clerk');
INSERT INTO emp2 VALUES(20, 'Bookkeeper'); -- 10-char job title
INSERT INTO emp2 VALUES(30, 'Analyst');
INSERT INTO emp2 VALUES(30, 'Analyst');
Comit;
DECLARE
TYPE NumList IS TABLE OF NUMBER;
depts NumList := NumList(10, 20, 30);
BEGIN
FORALL j IN depts.FIRST..depts.LAST
UPDATE emp2 SET job = job || ' (temp)'
WHERE deptno = depts(j);
-- raises a "value too large" exception
EXCEPTION
WHEN OTHERS THEN
COMMIT;
END;
/
PL/SQL procedure successfully completed
SQL> select * from emp2;
DEPTNO JOB
---------- ---------------
10 Clerk temp
10 Clerk temp
20 Bookkeeper
30 Analyst
30 Analyst
上邊的例子SQL引擎執行UPDATE語句3次,指定範圍内的每個索引号一次。第一個(depts(10))執行成功,但是第二個(depts(20))執行失敗(插入值超過了列長),是以,僅僅第二個執行被復原。
當執行任何SQL語句引發異常時,FORALL語句中斷(halt)。上邊的例子中,執行第二個UPDATE語句引發異常, 是以第三個語句不會執行。
三、用%BULK_ROWCOUNT 屬性計算FORALL疊代影響行數
在進行SQL資料操作語句時,SQL引擎打開一個隐式遊标(命名為SQL),該遊标的标量屬性(scalar attribute)有 %FOUND, %ISOPEN, %NOTFOUND, and %ROWCOUNT。
FORALL語句除具有上邊的标量屬性外,還有個複合屬性(composite attribute):%BULK_ROWCOUNT,該屬性具有索引表(index-by table)文法。它的第i個元素存貯SQL語句(INSERT, UPDATE或DELETE)第i個執行的處理行數。如果第i個執行未影響行,%bulk_rowcount (i),傳回0。FORALL與%bulk_rowcount屬性使用相同下标。例如:
DECLARE
TYPE NumList IS TABLE OF NUMBER;
depts NumList := NumList(10, 20, 50);
BEGIN
FORALL j IN depts.FIRST..depts.LAST
UPDATE emp SET sal = sal * 1.10 WHERE deptno = depts(j);
-- Did the 3rd UPDATE statement affect any rows?
IF SQL%BULK_ROWCOUNT(3) = 0 THEN ...
END;
%ROWCOUNT 傳回SQL語句所有執行處理總的行數
%FOUND和 %NOTFOUND僅與SQL語句的最後執行有關,但是,可以使用%BULK_ROWCOUNT推斷單個執行的值,如%BULK_ROWCOUNT(i)為0時,%FOUND和%NOTFOUND分别是FALSE和TRUE。
四、用%BULK_EXCEPTIONS屬性處理FORALL異常
在執行FORALL語句期間,PL/SQL提供一個處理異常的機制。該機制使批挷定(bulk-bind)操作能儲存異常資訊并繼續執行。方法是在FORALL語句中增加SAVE EXCEPTIONS關鍵字。文法為:
FORALL index IN lower_bound..upper_bound SAVE EXCEPTIONS
{insert_stmt | update_stmt | delete_stmt}
執行期間引發的所有異常都被儲存遊标屬性 %BULK_EXCEPTIONS中,它存貯一個集合記錄,每記錄有兩個字段:
%BULK_EXCEPTIONS(i).ERROR_INDEX:存貯在引發異常期間FORALL語句疊代(重複:iteration)
%BULK_EXCEPTIONS(i).ERROR_CODE:存貯相應的Oracle錯誤代碼
%BULK_EXCEPTIONS.COUNT存貯異常的數量。(該屬性不是%BULK_EXCEPTIONS集合記錄的字段)。如果忽略SAVE EXCEPTIONS,當引發異常時,FORALL語句停止執行。此時,SQL%BULK_EXCEPTIONS.COUNT 傳回1, 且SQL%BULK_EXCEPTIONS隻包含一條記錄。如果執行期間無異常 SQL%BULK_EXCEPTIONS.COUNT 傳回 0.例子:
DECLARE
TYPE NumList IS TABLE OF NUMBER;
num_tab NumList := NumList(10,0,11,12,30,0,20,199,2,0,9,1);
errors NUMBER;
dml_errors EXCEPTION;
PRAGMA exception_init(dml_errors, -24381);
BEGIN
FORALL i IN num_tab.FIRST..num_tab.LAST SAVE EXCEPTIONS
DELETE FROM emp WHERE sal > 500000/num_tab(i);
EXCEPTION
WHEN dml_errors THEN
errors := SQL%BULK_EXCEPTIONS.COUNT;
dbms_output.put_line('Number of errors is ' || errors);
FOR i IN 1..errors LOOP
dbms_output.put_line('Error ' || i || ' occurred during '||
'iteration ' || SQL%BULK_EXCEPTIONS(i).ERROR_INDEX);
dbms_output.put_line('Oracle error is ' ||
SQLERRM(-SQL%BULK_EXCEPTIONS(i).ERROR_CODE));
END LOOP;
END;
該例子中,當i等于2,6,10時,産生異常ZERO_DIVIDE,完成後SQL%BULK_EXCEPTIONS.COUNT為3,其值為(2,1476), (6,1476)和(10,1476),錯誤輸出如下:
Number of errors is 3
Error 1 occurred during iteration 2
Oracle error is ORA-01476: divisor is equal to zero
Error 2 occurred during iteration 6
Oracle error is ORA-01476: divisor is equal to zero
Error 3 occurred during iteration 10
Oracle error is ORA-01476: divisor is equal to zero
五、用BULK COLLECT子句取回查詢結果至集合中
在傳回到PL/SQL引擎之前,關鍵字BULK COLLECT告訴SQL引擎批挷定輸出集合。該關鍵字能用于SELECT INTO, FETCH INTO和RETURNING INTO語句中。文法如下:
... BULK COLLECT INTO collection_name[, collection_name] ...
示例1:
DECLARE
TYPE NumTab IS TABLE OF emp.empno%TYPE;
TYPE NameTab IS TABLE OF emp.ename%TYPE;
enums NumTab; -- no need to initialize
names NameTab;
BEGIN
SELECT empno, ename BULK COLLECT INTO enums, names FROM emp;
...
END;
示例2:
CREATE TYPE Coords AS OBJECT (x NUMBER, y NUMBER);
CREATE TABLE grid (num NUMBER, loc Coords);
INSERT INTO grid VALUES(10, Coords(1,2));
INSERT INTO grid VALUES(20, Coords(3,4));
DECLARE
TYPE CoordsTab IS TABLE OF Coords;
pairs CoordsTab;
BEGIN
SELECT loc BULK COLLECT INTO pairs FROM grid;
-- now pairs contains (1,2) and (3,4)
END;
示例3:
DECLARE
TYPE SalList IS TABLE OF emp.sal%TYPE;
sals SalList;
BEGIN
SELECT sal BULK COLLECT INTO sals FROM emp
WHERE ROWNUM <= 100;
...
END;
示例4:Examples of Bulk Fetching from a Cursor:
DECLARE
TYPE NameList IS TABLE OF emp.ename%TYPE;
TYPE SalList IS TABLE OF emp.sal%TYPE;
CURSOR c1 IS SELECT ename, sal FROM emp WHERE sal > 1000;
names NameList;
sals SalList;
BEGIN
OPEN c1;
FETCH c1 BULK COLLECT INTO names, sals; --可傳回到一個或多個集合
END;
示例5:Examples of Bulk Fetching from a Cursor:
DECLARE
TYPE DeptRecTab IS TABLE OF dept%ROWTYPE;
dept_recs DeptRecTab;
CURSOR c1 IS
SELECT deptno, dname, loc FROM dept WHERE deptno > 10;
BEGIN
OPEN c1;
FETCH c1 BULK COLLECT INTO dept_recs; --傳回到一個記錄(records)集合
END;
六、用LIMIT 子句限制批取出操作行
LIMIT子句可限制從資料庫中取出的行數。該子句僅能用于批(非标量 not scalar)FETCH語句.文法為:
FETCH ... BULK COLLECT INTO ... [LIMIT rows];
rows可以是文字的(literal),變量(variable)或表達式(expression),但必須傳回一個數字。否則,PL/SQL引發預定義異常VALUE_ERROR,如果為負數,PL/SQ引發INVALID_NUMBER。如果需要,PL/SQL四舍五入(round)為最近的整數.
下例中,在每個循環疊代中,FETCH語句傳回到索引表empnos中,先前的值會被覆寫。
DECLARE
TYPE NumTab IS TABLE OF NUMBER INDEX BY BINARY_INTEGER;
CURSOR c1 IS SELECT empno FROM emp;
empnos NumTab;
rows NATURAL := 10;
BEGIN
OPEN c1;
LOOP
FETCH c1 BULK COLLECT INTO empnos LIMIT rows;
EXIT WHEN c1%NOTFOUND;
...
END LOOP;
CLOSE c1;
END;
七、用RETURNING INTO子句取回DML結果至集合
你能在INSERT、UPDATE或DELETE語句的RETURNING INTO子句中使用BULK COLLECT子句。(注意,此處是沒有SELECT語句的)
DECLARE
TYPE NumList IS TABLE OF emp.empno%TYPE;
enums NumList;
BEGIN
DELETE FROM emp WHERE deptno = 20
RETURNING empno BULK COLLECT INTO enums;
-- if there were five employees in department 20,
-- then enums contains five employee numbers
END;
八、BULK COLLECT上的限制
1.You cannot bulk collect into an associative array that has a string type for the key. .
2.BULK COLLECT語句隻能用于伺服器端(server-side),而非用戶端
3.在BULK COLLECT INTO語句中的所有目标必須是集合,如下例:
DECLARE
TYPE NameList IS TABLE OF emp.ename%TYPE;
names NameList;
salary emp.sal%TYPE;
BEGIN
SELECT ename, sal BULK COLLECT INTO names, salary -- illegal target
FROM emp WHERE ROWNUM < 50;
...
END;.
4.複合目标(如對象)不能用于RETURNING INTO子句中.
5.當需要隐式資料類型轉換時,多複合目标(如對象集合)不能用于bulk collect into子句。
九、同時使用FORALL 和BULK COLLECT
Using FORALL and BULK COLLECT Together
你能使FORALL語句與BULK COLLECT結合,如下例:如果集合depts有3個元素,每個元素導緻5行被删除,則語句完成時,集合enums有15個元素:
FORALL j IN depts.FIRST..depts.LAST
DELETE FROM emp WHERE empno = depts(j)
RETURNING empno BULK COLLECT INTO enums;
注意:不能在FORALL語句中使用SELECT ... BULK COLLECT語句。
十、Using Host Arrays with Bulk Binds
用戶端程式能用匿名PL/SQL塊批挷定輸入和輸出數組(arrays)。實際上,這是與伺服器端傳遞集合最有效的方式。
Host arrays are declared in a host environment such as an OCI or Pro*C program and must be prefixed with a colon to distinguish them from PL/SQL collections. In the example below, an input host array is used in a DELETE statement. At run time, the anonymous PL/SQL block is sent to the database server for execution.
DECLARE
...
BEGIN
-- assume that values were assigned to the host array
-- and host variables in the host environment
FORALL i IN :lower..:upper
DELETE FROM emp WHERE deptno = :depts(i);
...
END;
限制:以下限制将應用于FORALL語句:
1. 在FORALL循環中,UPDATE語句中的SET子句和WHERE子句中不能指向同一個集合,你可能需要獲得另一個集合副本,以在WHERE子句指向新的名稱。
2. INSERT,UPDATE或DELETE語句必須至少涉及一個集合。如在LOOP插入一組常量的FORALL語句将引發異常。( FORALL的INSERT之類的語句一定要用in-bind 方式,如:
SQL> BEGIN
2 FORALL i IN 1..100
3 INSERT INTO parts VALUES(i,i);
4 END;
5 /
ORA-06550: 第 3 行, 第 5 列:
PLS-00435: 沒有 BULK In-BIND 的 DML 語句在 FORALL 内不能使用
ORA-06550: 第 2 行, 第 12 列:
PL/SQL: Statement ignored
3. 指定範圍内的所有集合元素必須存在,如果一進制素丢失或删除,你将收到一個錯誤,如:
DECLARE
TYPE NumList IS TABLE OF NUMBER;
depts NumList := NumList(10, 20, 30, 40);
BEGIN
depts.DELETE(3); -- delete third element
FORALL i IN depts.FIRST..depts.LAST
DELETE FROM emp WHERE deptno = depts(i); -- causes an error
END;
4. 下例顯示,複合值的輸入集合不能被分解和跳躍資料庫列
CREATE TABLE coords (x NUMBER, y NUMBER);
CREATE TYPE Pair AS OBJECT (m NUMBER, n NUMBER);
DECLARE
TYPE PairTab IS TABLE OF Pair;
pairs PairTab := PairTab(Pair(1,2), Pair(3,4), Pair(5,6));
TYPE NumTab IS TABLE OF NUMBER;
nums NumTab := NumTab(1, 2, 3);
BEGIN
FORALL i IN 1..3
UPDATE coords SET (x, y) = pairs(i)
WHERE x = nums(i);
END;
The workaround is to decompose the composite values manually:
DECLARE
TYPE PairTab IS TABLE OF Pair;
pairs PairTab := PairTab(Pair(1,2), Pair(3,4), Pair(5,6));
TYPE NumTab IS TABLE OF NUMBER;
nums NumTab := NumTab(1, 2, 3);
BEGIN
FORALL i in 1..3
UPDATE coords SET (x, y) = (pairs(i).m, pairs(i).n)
WHERE x = nums(i);
END;
5. 集合下标不能是表達式,示例:
FORALL j IN mgrs.FIRST..mgrs.LAST
DELETE FROM emp WHERE mgr = mgrs(j+1); -- invalid subscript
6. 遊标屬性%BULK_ROWCOUNT不能配置設定給其它集合,同樣,它也不能作為參數傳遞到子程式。