FOR循环与FORALL的性能比较

通常在SQL语句中给PL/SQL变量赋值叫做绑定（Binding）,一次绑定一个完整的集合称为批量绑定（Bulk Binding）。

批量绑定（Bulk binds）可以通过减少在PL/SQL和SQL引擎之间的上下文切换(context switches )提高了性能.

批量绑定（Bulk binds）包括：
(i) Input collections, use the FORALL statement，一般用来改善DML（INSERT、UPDATE和DELETE) 操作的性能
(ii) Output collections, use BULK COLLECT clause，一般用来提高查询（SELECT）的性能

FORALL的语法如下：

FORALL index IN lower_bound..upper_bound sql_statement;

下面是一个简单测试，用以说明FORALL与FOR循环的性能差异。
SQL> drop table blktest;

Table dropped.

Elapsed: 00:00:00.13
SQL>
SQL> CREATE TABLE blktest (num NUMBER(20), name varchar2(50));

Table created.

Elapsed: 00:00:00.08
SQL>
SQL> CREATE OR REPLACE PROCEDURE bulktest
2 IS
3 TYPE numtab IS TABLE OF NUMBER (20)
4 INDEX BY BINARY_INTEGER;
5
6 TYPE nametab IS TABLE OF VARCHAR2 (50)
7 INDEX BY BINARY_INTEGER;
8
9 pnums numtab;
10 pnames nametab;
11 t1 NUMBER;
12 t2 NUMBER;
13 t3 NUMBER;
14 BEGIN
15 FOR j IN 1 .. 1000000
16 LOOP
17 pnums (j) := j;
18 pnames (j) := 'Seq No. ' || TO_CHAR (j);
19 END LOOP;
20
21 SELECT DBMS_UTILITY.get_time
22 INTO t1
23 FROM DUAL;
24
25 FOR i IN 1 .. 1000000
26 LOOP
27 INSERT INTO blktest
28 VALUES (pnums (i), pnames (i));
29 END LOOP;
30
31 SELECT DBMS_UTILITY.get_time
32 INTO t2
33 FROM DUAL;
34
35 FORALL i IN 1 .. 1000000
36 INSERT INTO blktest
37 VALUES (pnums (i), pnames (i));
38
39 SELECT DBMS_UTILITY.get_time
40 INTO t3
41 FROM DUAL;
42
43 DBMS_OUTPUT.put_line ('Execution Time (hsecs)');
44 DBMS_OUTPUT.put_line ('---------------------');
45 DBMS_OUTPUT.put_line ('FOR loop: ' || TO_CHAR (t2 - t1));
46 DBMS_OUTPUT.put_line ('FORALL: ' || TO_CHAR (t3 - t2));
47 END;
48 /

Procedure created.

Elapsed: 00:00:01.46
SQL> exec bulktest;
Execution Time (hsecs)
---------------------
FOR loop: 30361
FORALL: 4792

PL/SQL procedure successfully completed.

Elapsed: 00:06:32.92

我们可以看到FORALL较FOR循环性能大大提高。

forall一般用于数组赋初值。跟do差不多的用法（其实也就是个do）。上个例子就知道了：
forall (i = 1:10, j = 1:10, b(i,j) /= 0.0)
a(i, j) = real(i + j)
b(i, j) = a(i,j) + b(i,j) * real(i*j)
end forall