Fetch out of Sequence error isn't returned when doing a single fetch

Question

Hi all,
As per my understanding, if we fetch all the rows from a sys_refcursor into a collection, within the pl/sql block, then it doesn't return any row to the calling program. Instead a fetch out of sequence error is encountered. This worked well. However, when I use a single fetch statement(in case the sys_refcursor contains a single row), I do not encounter "Fetch Out Of Sequence" error. It would be really helpful if anyone could please explain this behaviour.
Below is a sample code snippet that I ran in PL/SQL Developer:
CREATE OR REPLACE PROCEDURE test_cur_1(in_dept_id VARCHAR2,
out_cursor OUT SYS_REFCURSOR) IS
temp_count NUMBER;
temp_rec dept%ROWTYPE;
BEGIN
--This piece of code runs into a "Fetch Out Of Sequence" error, when trying to open up the sys_refcursor.
OPEN out_cursor FOR
SELECT *
FROM dept
WHERE dept_id = in_dept_id;--dept_id is the primary key
LOOP
EXIT WHEN out_cursor%NOTFOUND;
FETCH out_cursor
INTO temp_rec;
dbms_output.put_line('department name: '||temp_rec.dept_name);
END LOOP;
EXCEPTION
WHEN OTHERS THEN
OPEN out_cursor FOR
SELECT SQLCODE||'->'||SQLERRM FROM dual;
END;
CREATE OR REPLACE PROCEDURE test_cur_2(in_dept_id VARCHAR2,
out_cursor OUT SYS_REFCURSOR) IS
temp_count NUMBER;
temp_rec dept%ROWTYPE;
BEGIN
--This piece of code does not run into a "Fetch Out Of Sequence" error. Instead the output sys_refcursor opens up for no rows to display.
OPEN out_cursor FOR
SELECT *
FROM dept
WHERE dept_id = in_dept_id;--dept_id is the primary key
FETCH out_cursor
INTO temp_rec;
dbms_output.put_line('department name: '||temp_rec.dept_name);
EXCEPTION
WHEN OTHERS THEN
OPEN out_cursor FOR
SELECT SQLCODE||'->'||SQLERRM FROM dual;
END;
Appreciate all your time!

Answer 1

When you loop through a cursor, Oracle does not know how many rows there will be in the cursor until it has fetched them all, and it cannot know it has fetched them all until it does one last fetch after getting the last row to find that ti gets nothing back. In psuedo code the fetch command works something like:
open cursor
cursor%notfound := null;
-- This is the fetch phase
ask sql engine to give me a row
if I got one then
   cursor%notfound := false;
   populate variables/record with values
else
   cursor%notfound := true;
end if;
-- end fetch phase
go onto next command in the pl/sql blockSo, in your first procedure you fetch the last row of the cursor then go back to the top of the loop. Since Oracle does not know at this point that the last row has already been fetched so, out_cursor%notfound is still false, so it does another fetch at which point it now "knows" that the last row is fetched so notfound becomes true and your loop exits, returning control to the caller. At this point, presumably the caller attempts to fetch from the cursor. Since the cursor handle is already pointing past the last row you get the fetch out of sequence.
In your second procedure, you fetch a single row from the cursor. You know that the cursor is pointing at the "last" row since you are querying based on the PK, but Oracle does not know that yet. You now return that cursor to the caller who does one more fetch which moves the pointer past the end of the cursor. The caller "sees" no rows, but that is because your procedure already fetched the rows, but did not "check" for the last row.
John