How to check whether a dynamic cursor will retrieve no records? - oracle

Question

I have a procedure which has a structure as given below:
PROCEDURE broker(prm_qgent in varchar2, prm_cursor out sys_refcursor)
IS
mmy_query varchar(200);
BEGIN
OPEN prm_cursor FOR SELECT * FROM DUAL;
mmy_query :='SELECT *some dynamic query* where 1=1';
if prm_agent is not null then
mmy_query := mmy_query ||'AND agent_code = ''' ||prm_agent || '''';
end if;
OPEN prm_cursor FOR mmy_query;
END broker;
mmy_query is the search criteria. So if a broker is not in the table, it should retrieve zero records.
mmy_query is the dynamic query to retrieve records. If dynamic query retrieves record, it will only retrieve 1 record. So, I want to check if mmy_query retrieves no records.
tried prm_cursor%ROWTYPE, shows zero record in both cases.
tried SQL%ROWTYPE, always shows 1 record.

Answer 1

You need to open the cursor, fetch the first record from it and check FOUND or NOT_FOUND attributtes.
Note: if the first row is fetched, then there is no other way to "rewind the cursor to the beginning" except that closing that cursor and opening it again (and executing the same query again).
According to the documentation (from the above link), each named cursor has 4 attributtes:
%ISOPEN
named_cursor%ISOPEN has the value TRUE if the cursor is open, and
FALSE if it is not open.
%FOUND
named_cursor%FOUND has one of these values:
If the cursor is not open, INVALID_CURSOR
If cursor is open but no fetch was tried, NULL.
If the most recent fetch returned a row, TRUE.
If the most recent fetch did not return a row, FALSE.
%NOTFOUND
named_cursor%NOTFOUND has one of these values:
If cursor is not open, INVALID_CURSOR.
If cursor is open but no fetch was tried, NULL.
If the most recent fetch returned a row, FALSE.
If the most recent fetch did not return a row, TRUE.
%ROWCOUNT
named_cursor%ROWCOUNT has one of these values:
If cursor is not open, INVALID_CURSOR.
If cursor is open, the number of rows fetched so far.
As you see, there is no way to check whether the query returned either no rows or some rows whithout opening the cursor and fetching from it.
Simple examples:
A query that returns some rows:
DECLARE
my_cursor SYS_REFCURSOR;
y VARCHAR(10);
BEGIN
OPEN my_cursor FOR 'select * FROM dual WHERE 1=1';
FETCH my_cursor INTO y;
IF my_cursor%found THEN
DBMS_OUTPUT.PUT_LINE('FOUND');
ELSE
DBMS_OUTPUT.PUT_LINE('NOT FOUND');
END IF;
CLOSE my_cursor;
END;
/
Result: FOUND
A query that returns empty resultset:
DECLARE
my_cursor SYS_REFCURSOR;
y VARCHAR(10);
BEGIN
OPEN my_cursor FOR 'select * FROM dual WHERE 1=0';
FETCH my_cursor INTO y;
IF my_cursor%found THEN
DBMS_OUTPUT.PUT_LINE('FOUND');
ELSE
DBMS_OUTPUT.PUT_LINE('NOT FOUND');
END IF;
CLOSE my_cursor;
END;
/
Result: NOT FOUND

Answer 2

It is unknown whether or not rows will be found until the caller attempts to fetch from the cursor. At the time when the cursor is opened, this has not happened yet. A procedure that simply opens and returns a cursor has no way of knowing what will happen when the caller tries to fetch from it after the procedure has completed.
Perhaps it would be theoretically possible to parse the execution plan from v$sql_plan to get the estimated cardinality, if you could find its sql_id (perhaps from querying v$sql), but this would not be straightforward and at best the result would be a guess.
All you can really do is execute the query twice (or execute a simplified version of it, if that's possible), bearing in mind that the results could change between executions if other sessions are modifying the data.