Calling a PostgreSQL function from Java

Question

I have written a function that I would like to call in Java. But I don't think it is able to do anything with the query that I passed. Following is my code from java:

String QUERY_LOCATION = "select (license_plate) as test from carInst( (select     category_name from reservation where rid = ?) , (select lname from reservation where rid =  ?))";  
        //PreparedStatement check_location = null;
        PreparedStatement check_location = connection.prepareStatement(QUERY_LOCATION);
        check_location.setInt(1, rid);
        check_location.setInt(2, rid);
        rs = check_location.executeQuery();
        if (rs.next()) {
            System.out.print("Car found: "+rs.getString("test")+"\n");
            license_plate = rs.getString("test");
            update_reservation.setString(5, license_plate);
            
            bool = false;
        } else {
            System.out
                    .print("There is no car available\n");
        }

And following is my stored procedure written in PL/pgSQL (PostgreSQL):

CREATE OR REPLACE FUNCTION carInst(cname varchar(20), loc varchar(20) ) 
RETURNS TABLE (license_plate varchar(6) ) AS $$
BEGIN 
    DECLARE cur CURSOR 
        FOR SELECT carinstance.license_plate, carmodel.category_name, carinstance.lname FROM carinstance,carmodel 
            WHERE carinstance.mid = carmodel.mid ;                  
BEGIN 
    FOR rec IN cur LOOP
        RETURN QUERY SELECT distinct carinstance.license_plate FROM Carinstance 
            WHERE rec.category_name  = cname
                AND rec.lname = loc
                AND rec.license_plate=carinstance.license_plate;
        END LOOP;
    END; 
END;
$$ LANGUAGE plpgsql;

When I run the code in Java, the print statement prints a null value for Car found. I would really appreciate some help here.

Answer 1

Problems

• Most importantly, the query in the LOOP is nonsense. You select rows from carinstance, but all conditions are on rec. This select all rows multiple times.

• One END too many. FOR has no END, only LOOP has.

• Whenever you feel the temptation to work with an explicit cursor in plpgsql, stop right there. Chances are, you are doing it wrong. A FOR loop has an implicit cursor anyway.

• Don't mess with mixed case identifiers without double quotes. I converted all identifiers to lower case.

• You use one simple query, spread out over a cursor and another query. This can all be much simpler.

Solution

Try this simple SQL function instead:

CREATE OR REPLACE FUNCTION car_inst(_cname text, _loc text) 
  RETURNS TABLE (license_plate text)
  LANGUAGE sql AS
$func$
SELECT DISTINCT ci.license_plate
FROM   carmodel    cm
JOIN   carinstance ci USING (mid)
WHERE  cm.category_name  = $1
AND    ci.lname = $2
$func$;

Call:

SELECT license_plate AS test FROM car_inst(
         (SELECT category_name FROM reservation WHERE rid = ?)
       , (SELECT lname FROM reservation WHERE rid = ?)
       );  

Or build it all into your function:

CREATE OR REPLACE FUNCTION car_inst(_cname text, _loc text) 
  RETURNS TABLE (license_plate text)
  LANGUAGE sql AS
$func$
SELECT DISTINCT ci.license_plate
FROM   carmodel    cm
JOIN   carinstance ci USING (mid)
JOIN   reservation r1 ON r1.category_name = cm.category_name
JOIN   reservation r2 ON r2.lname = ci.lname
WHERE  r1.rid = $1
AND    r2.rid = $2;
$func$;

Call:

"SELECT license_plate AS test FROM car_inst(? , ?)";  

Remember: The OUT parameter license_plate is visible anywhere in the body of the function. Therefore you must table-qualify the column of the same name at all times to prevent a naming collision.

DISTINCT may or may not be redundant.