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;
oracle批量绑定forall bulk collect
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,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
10g开始forall语句可以使用三种方式:
i in low..up
i in indices of collection 取得集合元素下标的值
i in values of collection 取得集合元素的值
forall语句还可以使用部分集合元素
sql%bulk_rowcount(i)表示forall语句第i元素所作用的行数
CREATE TABLE parts1 (pnum INTEGER, pname VARCHAR2(15));
CREATE TABLE parts2 (pnum INTEGER, pname VARCHAR2(15));
CREATE TABLE parts3 (pnum INTEGER, pname VARCHAR2(15));
CREATE TABLE parts4 (pnum INTEGER, pname VARCHAR2(15));
set serveroutput on --把屏幕显示开关置上
DECLARE
TYPE NumTab IS TABLE OF parts1.pnum%TYPE INDEX BY PLS_INTEGER;
TYPE NameTab IS TABLE OF parts1.pname%TYPE INDEX BY PLS_INTEGER;
pnums NumTab;
pnames NameTab;
iterations CONSTANT PLS_INTEGER := 50000;
t1 INTEGER; t2 INTEGER; t3 INTEGER; t4 INTEGER; t5 INTEGER;
stmt_str varchar2(255);table_name varchar2(255);
BEGIN
FOR j IN 1..iterations LOOP -- load index-by tables
pnums(j) := j;
pnames(j) := 'Part No. ' || TO_CHAR(j);
END LOOP;
t1 := dbms_utility.get_time;
FOR i IN 1..iterations LOOP -- use FOR loop
INSERT INTO parts1 VALUES (pnums(i), pnames(i));
END LOOP;
t2 := dbms_utility.get_time;
FORALL i IN 1..iterations -- use FORALL statement
INSERT INTO parts2 VALUES (pnums(i), pnames(i));
t3 := dbms_utility.get_time;
table_name:='parts3';
stmt_str := 'INSERT INTO ' || table_name || ' values (:num, :pname)';
FOR i IN 1..iterations LOOP -- use FORALL statement
EXECUTE IMMEDIATE stmt_str USING pnums(i), pnames(i);
END LOOP;
t4 := dbms_utility.get_time;
table_name:='parts4';
stmt_str := 'INSERT INTO ' || table_name || ' values (:num, :pname)';
FORALL i IN 1..iterations-- use FORALL statement
EXECUTE IMMEDIATE stmt_str USING pnums(i), pnames(i);
t5 := dbms_utility.get_time;
dbms_output.put_line('Execution Time (secs)');
dbms_output.put_line('---------------------');
dbms_output.put_line('FOR loop: ' || TO_CHAR((t2 - t1)/100));
dbms_output.put_line('FORALL: ' || TO_CHAR((t3 - t2)/100));
dbms_output.put_line('FOR loop: ' || TO_CHAR((t4 - t3)/100));
dbms_output.put_line('FORALL: ' || TO_CHAR((t5 - t4)/100));
COMMIT;
END;
/
DROP TABLE parts1;
DROP TABLE parts2;
DROP TABLE parts3;
DROP TABLE parts4;
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。现在,我们对该特性进行一些简单的测试和分析。
1. 首先,我们创建一个表,并插入100000条记录
在SQL/Plus中执行下列脚本:
drop table empl_tbl
/
create table empl_tbl(last_name varchar2(20),
first_name varchar2(10),
salary number(10))
/
begin
for i in 3000..102999 loop
insert into empl_tbl(last_name,first_name,salary) values('carl'||(i-3000),'wu'||(103000-i),i);
end loop;
end;
/
commit
/
select count(*) from empl_tbl;
/
2. 使用三种方法计算表中某一字段含有多少个不重复值
2.1 使用常规的Distinct来实现
SQL> select count(distinct last_name) "Distinct Last Name" from empl_tbl;
Distinct Last Name
------------------
100000
Executed in 0.36 seconds
我们可以看到,常规方法需要0.36秒查出该表中有100000个不重复的Last_name值。
2.2 使用游标来实现
我们执行下面语句来统计Last_name字段的不重复值个数:
declare
all_rows number(10);
temp_last_name empl_tbl.last_name%type;
begin
all_rows:=0;
temp_last_name:=' ';
for cur in (select last_name from empl_tbl order by last_name) loop
if cur.last_name!=temp_last_name then
all_rows:=all_rows+1;
end if;
temp_last_name:=cur.last_name;
end loop;
dbms_output.put_line('all_rows are '||all_rows);
end;
请注意上面代码中的黑体部分使用了一个For Loop游标,为了提高程序可读性,我们没有显示定义游标变量。
执行结果:
all_rows are 100000
PL/SQL procedure successfully completed
Executed in 1.402 seconds
游标需要1.4秒才能查出该表中有100000个不重复的Last_name值,所耗时间是Distinct查询的3倍多。
2.3 使用Bulk Collect批查询来实现
示例代码如下:
declare
all_rows number(10);
--首先,定义一个Index-by表数据类型
type last_name_tab is table of empl_tbl.last_name%type index by binary_integer;
last_name_arr last_name_tab;
--定义一个Index-by表集合变量
temp_last_name empl_tbl.last_name%type;
begin
all_rows:=0;
temp_last_name:=' ';
--使用Bulk Collect批查询来充填集合变量
select last_name bulk collect into last_name_arr from empl_tbl;
for i in 1..last_name_arr.count loop
if temp_last_name!=last_name_arr(i) then
all_rows:=all_rows+1;
end if;
temp_last_name:=last_name_arr(i);
end loop;
dbms_output.put_line('all_rows are '||all_rows);
end;
请注意上面代码中,我们首先定义了一个Index-by表数据类型last_name_tab,然后定义了一个该集合数据类型的变量last_name_arr,最后我们使用Bulk Collect批查询来充填last_name_arr,请注意它的使用语法。
执行结果:
all_rows are 100000
PL/SQL procedure successfully completed
Executed in 0.28 seconds
从上面执行结果,我们可以看到,Bulk Collect批查询只需要0.28秒就能查出该表中有100000个不重复的Last_name值,所耗时间只有游标查询的1/5,同时它比Distinct常规查询的速度也要快。