今天学习openGauss定义游标
为了处理SQL语句，存储过程进程分配一段内存区域来保存上下文联系，游标是指向上下文区域的句柄或指针。借助游标，存储过程可以控制上下文区域的变化。
1.准备数据
create schema tpcds;
CREATE TABLE tpcds.reason
(
r_reason_sk integer,
r_reason_id character(16),
r_reason_desc character(100)
);
INSERT INTO tpcds.reason values(3,‘AAAAAAAABAAAAAAA’,‘reason 1’),
(10,‘AAAAAAAABAAAAAAA’,‘reason 2’),(4,‘AAAAAAAABAAAAAAA’,‘reason 3’),
(10,‘AAAAAAAABAAAAAAA’,‘reason 4’),(10,‘AAAAAAAABAAAAAAA’,‘reason 5’),
(20,‘AAAAAAAACAAAAAAA’,‘reason 6’),(30,‘AAAAAAAACAAAAAAA’,‘reason 7’);
create table company(name varchar(100), loc varchar(100), no integer);
insert into company values (‘macrosoft’, ‘usa’, 001);
insert into company values (‘oracle’, ‘usa’, 002);
insert into company values (‘backberry’, ‘canada’, 003);
2. SELECT语句，用一个游标读取一个表
–开始一个事务
start transaction;
–建立一个名为cursor1的游标。
CURSOR cursor1 FOR SELECT * FROM tpcds.reason ORDER BY 1;
–在系统视图pg_cursors中查看可用游标
select * from pg_cursors;
–抓取头3行到游标cursor1里
FETCH FORWARD 3 FROM cursor1;
–从当前关联位置开始，抓取前面的1行
FETCH BACKWARD 1 FROM cursor1;
–关闭游标并提交事务
CLOSE cursor1;
select * from pg_cursors;
end;
3. VALUES子句，用一个游标读取VALUES子句中的内容
–建立一个名为cursor2的游标
start transaction;
CURSOR cursor2 FOR VALUES(1,2),(0,3) ORDER BY 1;
FETCH FORWARD 2 FROM cursor2;
CLOSE cursor2;
end;
4. WITH HOLD游标的使用
–声明该游标在创建它的事务结束后仍可继续使用
DECLARE cursor1 CURSOR WITH HOLD FOR SELECT * FROM tpcds.reason ORDER BY 1;
–抓取接下来的3行
FETCH FORWARD 3 FROM cursor1;
CLOSE cursor1;
5.移动游标
START TRANSACTION;
CURSOR cursor1 FOR SELECT * FROM tpcds.reason ORDER BY 1;
–忽略游标cursor1的前3行
MOVE FORWARD 3 FROM cursor1;
–抓取游标cursor1的前4行
FETCH 4 FROM cursor1;
CLOSE cursor1;
end;
6.存储过程中使用游标
create or replace procedure test_cursor_1
as
company_name varchar(100);
company_loc varchar(100);
company_no integer;
cursor c1_all is --cursor without args
select name, loc, no from company order by 1, 2, 3;
begin
if not c1_all%isopen then
open c1_all;
end if;
loop
fetch c1_all into company_name, company_loc, company_no;
RAISE INFO ‘company_name: %’ ,company_name;
exit when c1_all%notfound;
end loop;
if c1_all%isopen then
close c1_all;
end if;
end;
/
call test_cursor_1();
drop procedure test_cursor_1;
7.清理数据
drop schema tpcds cascade;
drop table company;
课后作业
1.创建游标，且使用select子句指定游标返回的行，分别使用FETCH抓取数据，MOVE重定位游标
omm=# create schema my_schema;
CREATE SCHEMA
omm=#
omm=# create table my_schema.product
omm-# (
omm(# id integer,
omm(# name char(30)
omm(# );
CREATE TABLE
omm=#
omm=# insert into my_schema.product values(1,‘apple’),
omm-# (2,‘orange’),(3,‘banana’),(4,‘juice’),(5,‘pear’),
omm-# (6,‘grape’),(7,‘mango’);
INSERT 0 7
omm=#
omm=# select * from my_schema.product;
id | name
----±-------------------------------
1 | apple
2 | orange
3 | banana
4 | juice
5 | pear
6 | grape
7 | mango
(7 rows)
omm=# start transaction;
START TRANSACTION
omm=# cursor cur for select * from my_schema.product order by 1;
DECLARE CURSOR
omm=# fetch forward 2 from cur;
id | name
----±-------------------------------
1 | apple
2 | orange
(2 rows)
omm=# move forward 2 from cur;
MOVE 2
omm=# fetch forward 2 from cur;
id | name
----±-------------------------------
5 | pear
6 | grape
(2 rows)
2.在系统视图pg_cursors中查看游标
omm=# select * from pg_cursors;
name | statement | is_holdable | is_binary | is_scrollable | creation_time
------±-----------------------------------------------------------±------------±----------±--------------±------------------------------
cur | cursor cur for select * from my_schema.product order by 1; | f | f | t | 2021-12-22 10:14:53.452979+08
(1 row)
omm=# close cur;
CLOSE CURSOR
omm=# end;
COMMIT
omm=# select * from pg_cursors;
name | statement | is_holdable | is_binary | is_scrollable | creation_time
------±----------±------------±----------±--------------±--------------
(0 rows)
3.创建一个使用游标的存储过程
omm=# select * from my_schema.product;
id | name
----±-------------------------------
1 | apple
2 | orange
3 | banana
4 | juice
5 | pear
6 | grape
7 | mango
(7 rows)
omm=# create or replace procedure test_cur
omm-# as
omm# product_id integer; omm# product_name char(30);
omm# cursor test_cur is --cursor without args omm# select id, name from my_schema.product order by 1, 2;
omm# begin omm# if not test_cur%isopen then
omm# open test_cur; omm# end if;
omm# loop omm# fetch test_cur into product_id, product_name;
omm# raise info ‘product_name: %’ ,product_name; omm# exit when test_cur%notfound;
omm# end loop; omm# if test_cur%isopen then
omm# close test_cur; omm# end if;
omm# end; omm# /
CREATE PROCEDURE
omm=# call test_cur();
INFO: product_name: apple
INFO: product_name: orange
INFO: product_name: banana
INFO: product_name: juice
INFO: product_name: pear
INFO: product_name: grape
INFO: product_name: mango
INFO: product_name: mango
test_cur
(1 row)
4.清理数据
omm=# drop procedure test_cur;
DROP PROCEDURE
omm=# drop schema my_schema cascade;
NOTICE: drop cascades to table my_schema.product
DROP SCHEMA
omm=#
