PostgreSQL 游標的種類

1.說明：

1）cursor和refcursor的區別：
靜態游標，不能修改里面的語句
動態游標，可以多次open和close，指定不同的SQL
隱式游標，DML和for操作，都在內部轉換為了一個隱式游標在執行

2）fetch的區別：
使用fetch cur 和 fetch next from cur 一樣，因為NEXT 是 FETCH 的默認選項.

2.創建測試表：

create table test(id int, info text, crt_time timestamp);
insert into test select generate_series(1,10), 'test', now();

　　

3.測試：

DO
$$
DECLARE
id integer;
one_row record;
ret_cur CURSOR FOR SELECT * FROM test; --靜態游標，不能修改里面的語句
ret_cur2 refcursor;--動態游標，可以多次open和close，指定不同的SQL
BEGIN
id:=1;
raise notice 'test id: %', id;

--動態游標
OPEN ret_cur2 FOR SELECT * FROM test;
FETCH ret_cur2 INTO one_row;
while ret_cur2%FOUND loop
raise notice 'row info, id is: %, info is: %, crt_time is: %', one_row.id, one_row.info, one_row.crt_time;
FETCH ret_cur2 INTO one_row;
END loop;
CLOSE ret_cur2;

raise notice 'user fetch next: ';

--靜態游標：
OPEN ret_cur;
FETCH NEXT FROM ret_cur INTO one_row;
while ret_cur%FOUND loop
raise notice 'row info, id is: %, info is: %, crt_time is: %', one_row.id, one_row.info, one_row.crt_time;
FETCH NEXT FROM ret_cur INTO one_row;
END loop;
CLOSE ret_cur;


--隱式游標，實際上select * from test是存在一個內部的游標中的：
raise notice '隱式游標：';
FOR one_row IN (SELECT * FROM test) loop
raise notice 'row info, id is: %, info is: %, crt_time is: %', one_row.id, one_row.info, one_row.crt_time;
END loop;

--最后執行：
EXCEPTION WHEN NO_DATA_FOUND THEN 
raise notice 'no data FOUND';

END 
$$

　　

4.執行結果：

00000: test id: 1
00000: row info, id is: 1, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 2, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 3, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 4, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 5, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 6, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 7, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 8, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 9, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 10, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: user fetch next: 
00000: row info, id is: 1, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 2, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 3, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 4, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 5, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 6, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 7, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 8, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 9, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 10, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: 隱式游標：
00000: row info, id is: 1, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 2, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 3, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 4, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 5, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 6, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 7, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 8, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 9, info is: test, crt_time is: 2020-09-09 15:01:35.837492
00000: row info, id is: 10, info is: test, crt_time is: 2020-09-09 15:01:35.837492

　　

5.游標屬性：

%FOUND： bool - TRUE if >1 row returned 
%NOTFOUND：bool - TRUE if 0 rows returned 
%ISOPEN： bool - TRUE if cursor still open 
%ROWCOUNT：int - number of rows affected by last SQL statement

注：NO_DATA_FOUND和%NOTFOUND的用法是有區別的，小結如下： 
1）SELECT . . . INTO 語句觸發 NO_DATA_FOUND； 
2）當一個顯式光標的 where 子句未找到時觸發 %NOTFOUND； 
3）當UPDATE或DELETE 語句的where 子句未找到時觸發 SQL%NOTFOUND； 
4）在光標的提取(Fetch)循環中要用 %NOTFOUND 或%FOUND 來確定循環的退出條件，不要用NO_DATA_FOUND