当Oracle运行PL/SQL时会使用两套引擎，所有procedural code由PL/SQL engine 完成，所有SQL由SQL engine处理。
所以假如Oracle从一个collection中循环执行相同的DML操作，那么为了避免两套engine切换所消耗的系统资源，可以使用bulk binds来把所有的DML操作binding到一次操作中完成。这将极大提高PL/SQL的执行效率。
以下是简单的测试，用两种方式插入100000条数据，可以看到效率提高了7倍左右。
代码:--------------------------------------------------------------------------------
SQL> CREATE TABLE test1(
2 id NUMBER(10),
3 description VARCHAR2(50));
Table created
SQL> ALTER TABLE test1 ADD (
2 CONSTRAINT test1_pk PRIMARY KEY (id));
Table altered
SQL> SET TIMING ON;
SQL> DECLARE
2 TYPE id_type IS TABLE OF test1.id%TYPE;
3 TYPE description_type IS TABLE OF test1.description%TYPE;
4
5 t_id id_type := id_type();
6 t_description description_type := description_type();
7 BEGIN
8 FOR i IN 1 .. 100000 LOOP
9 t_id.extend;
10 t_description.extend;
11
12 t_id(t_id.last) := i;
13 t_description(t_description.last) := 'Description: ' To_Char(i);
14 END LOOP;
15
16 FOR i IN t_id.first .. t_id.last LOOP
17 INSERT INTO test1 (id, description)
18 VALUES (t_id(i), t_description(i));
19 END LOOP;
20
21 COMMIT;
22 END;
23 /
PL/SQL procedure sUCcessfully completed
Executed in 141.233 seconds
SQL> truncate table test1;
Table truncated
Executed in 0.631 seconds
SQL>
SQL> DECLARE
2 TYPE id_type IS TABLE OF test1.id%TYPE;
3 TYPE description_type IS TABLE OF test1.description%TYPE;
4
5 t_id id_type := id_type();
6 t_description description_type := description_type();
7 BEGIN
8 FOR i IN 1 .. 100000 LOOP
9 t_id.extend;
10 t_description.extend;
11
12 t_id(t_id.last) := i;
13 t_description(t_description.last) := 'Description: ' To_Char(i);
14 END LOOP;
15
16 FORALL i IN t_id.first .. t_id.last
17 INSERT INTO test1 (id, description)
18 VALUES (t_id(i), t_description(i));
19
20 COMMIT;
21 END;
22 /
PL/SQL procedure successfully completed
Executed in 27.52 seconds
SQL> select count(*) from test1;
COUNT(*)
----------
100000
Executed in 0.04 seconds
SQL>--------------------------------------------------------------------------------
下面我们使用上面那个例子中插入的100000条数据，来测试一下BULK COLLECT的威力。
代码:--------------------------------------------------------------------------------
SQL> SET TIMING ON;
SQL>
SQL> DECLARE
2 TYPE id_type IS TABLE OF test1.id%TYPE;
3 TYPE description_type IS TABLE OF test1.description%TYPE;
4
5 t_id id_type := id_type();
6 t_description description_type := description_type();
7
8 CURSOR c_data IS
9 SELECT *
10 FROM test1;
11 BEGIN
12 FOR cur_rec IN c_data LOOP
13 t_id.extend;
14 t_description.extend;
15
16 t_id(t_id.last) := cur_rec.id;
17 t_description(t_description.last) := cur_rec.description;
18 END LOOP;
19 END;
20 /
PL/SQL procedure successfully completed
Executed in 2.974 seconds
SQL>
SQL> DECLARE
2 TYPE id_type IS TABLE OF test1.id%TYPE;
3 TYPE description_type IS TABLE OF test1.description%TYPE;
4
5 t_id id_type;
6 t_description description_type;
7 BEGIN
8 SELECT id, description
9 BULK COLLECT INTO t_id, t_description FROM test1;
10 END;
11 /
PL/SQL procedure successfully completed
Executed in 0.371 seconds
SQL>--------------------------------------------------------------------------------
结论：当我们需要将大量的检索结果放入一个collection的时候，使用bulking将比直接使用cursor循环有效的多。
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