oracle学习之bulk collect用法

通过bulk collect减少loop处理的开销，使用Bulk Collect提高Oracle查询效率

Oracle8i中首次引入了Bulk Collect特性，该特性可以让我们在PL/SQL中能使用批查询，批查询在某些情况下能显著提高查询效率。

采用bulk collect可以将查询结果一次性地加载到collections中。

而不是通过cursor一条一条地处理。

可以在select into,fetch into,returning into语句使用bulk collect。

注意在使用bulk collect时，所有的into变量都必须是collections.

 

举几个简单的例子：

--在select into语句中使用bulk collect

DECLARE
TYPE SalList IS TABLE OF emp.sal%TYPE;
sals SalList;
BEGIN
-- Limit the number of rows to 100.
SELECT sal BULK COLLECT INTO sals FROM emp
WHERE ROWNUM <= 100;
-- Retrieve 10% (approximately) of the rows in the table.
SELECT sal BULK COLLECT INTO sals FROM emp SAMPLE 10;

END;
/

--在fetch into中使用bulk collect

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;
END;
/

--在returning into中使用bulk collect

CREATE TABLE emp2 AS SELECT * FROM employees;
DECLARE
TYPE NumList IS TABLE OF employees.employee_id%TYPE;
enums NumList;
TYPE NameList IS TABLE OF employees.last_name%TYPE;
names NameList;
BEGIN
DELETE FROM emp2 WHERE department_id = 30
RETURNING employee_id, last_name BULK COLLECT INTO enums, names;
dbms_output.put_line('Deleted ' || SQL%ROWCOUNT || ' rows:');
FOR i IN enums.FIRST .. enums.LAST
LOOP
dbms_output.put_line('Employee #' || enums(i) || ': ' || names(i));
END LOOP;
END;
/
DROP TABLE emp2;

--在select into语句中使用bulk collect

DECLARE
TYPE SalList IS TABLE OF emp.sal%TYPE;
sals SalList;
BEGIN
-- Limit the number of rows to 100.
SELECT sal BULK COLLECT INTO sals FROM emp
WHERE ROWNUM <= 100;
-- Retrieve 10% (approximately) of the rows in the table.
SELECT sal BULK COLLECT INTO sals FROM emp SAMPLE 10;

END;
/

--在fetch into中使用bulk collect

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;
END;
/

--在returning into中使用bulk collect

CREATE TABLE emp2 AS SELECT * FROM employees;
DECLARE
TYPE NumList IS TABLE OF employees.employee_id%TYPE;
enums NumList;
TYPE NameList IS TABLE OF employees.last_name%TYPE;
names NameList;
BEGIN
DELETE FROM emp2 WHERE department_id = 30
RETURNING employee_id, last_name BULK COLLECT INTO enums, names;
dbms_output.put_line('Deleted ' || SQL%ROWCOUNT || ' rows:');
FOR i IN enums.FIRST .. enums.LAST
LOOP
dbms_output.put_line('Employee #' || enums(i) || ': ' || names(i));
END LOOP;
END;
/
DROP TABLE emp2;

 

通过上面的测试和分析，我们可以看到Bulk 
Collect批查询在某种程度上可以提高查询效率，它首先将所需数据读入内存，然后再统计分析，这样就可以提高查询效率。但是，如果Oracle数据库的内存较小，Shared 
Pool Size不足以保存Bulk Collect批查询结果，那么该方法需要将Bulk Collect的集合结果保存在磁盘上，在这种情况下，Bulk 
Collect方法的效率反而不如其他两种方法，有兴趣的读者可以进一步测试。

另外，除了Bulk Collect批查询外，我们还可以使用FORALL语句来实现批插入、删除和更新，这在大批量数据操作时可以显著提高执行效率。