Thread: PostgreSQL FUNCTION return problem

Hello, i have the following problem and i was wondering if you coulde help me. I'll try to describe it as in depth as i can.

   I have a FUNCTION in a postgresql database which subtracts two DOUBLE PRECISION FIELDS and returns the result. The problem is that when the result should be zero, the number i
get is a number ridiculously close to zero in scientific notation (for example 2.4697823124E-14) but not zero. I know (or at least i think i know) the problem lies within the way postgresql and Java communicate because if a connect through a console to the db and run the function i get the result right (zero) but if i debug the java code i get ther wrong non-zero-but-very-close result

this is the function:

CREATE OR REPLACE FUNCTION getSaldoParticipante(BIGINT) RETURNS DOUBLE PRECISION AS '
   DECLARE
    idParticipante ALIAS FOR $1;
    result DOUBLE PRECISION;
   BEGIN
  
   result := 0;

   Select (Select CASE WHEN saldo ISNULL then 0 else saldo end FROM (Select sum(PG.saldo) as saldo FROM pagos.pagos PG WHERE PG.saldo > 0 AND PG.participante=idParticipante AND PG.deleted = false) tmp)-(Select CASE WHEN saldo ISNULL then 0 else saldo end FROM (Select sum(DE.saldo) as saldo FROM pagos.deudas DE WHERE DE.deleted = false AND DE.participante=idParticipante) tmp) INTO result;
  
   RETURN result;

END;'
LANGUAGE 'plpgsql' CALLED ON NULL INPUT;

this is the Java code where i get the result that should be zero but instead is just very close to zero

        String query = "select getSaldoParticipante(?)";
        params.add(new ParameterValue(idParticipante, Types.BIGINT));
        
        try {
            conn = tx.getConnection();
            
            //preparo y ejecuto el statement
            pstmt = prepareStatement (conn, query, params);
            rs = pstmt.executeQuery();
            
            //itero los registros
            while (rs.next()) {
                //the problems appears at the following line
                result = rs.getDouble("getSaldoParticipante");
            }
        }
        catch(Exception e){
                 ...


the only thing that works here is to use rs.getInt instead of rs.getDouble, but that is not good enough since i dont want to truncate the decimal part of the number.

So, a lot of thanks in advance, any help will be greatly appreciated.

¡Saludos!

Rafael

Rafael Barrera Oro wrote:
> Hello, i have the following problem and i was wondering if you coulde help
> me. I'll try to describe it as in depth as i can.
>
>    I have a FUNCTION in a postgresql database which subtracts two DOUBLE
> PRECISION FIELDS and returns the result. The problem is that when the result
> should be zero, the number i
> get is a number ridiculously close to zero in scientific notation (for
> example 2.4697823124E-14) but not zero.

Start here:

http://www.google.com/search?q=floating+point+rounding

The short answer is - never expect the subtraction of two "equal"
floating point values to be zero. Instead of testing with equality, test
with a very small range.

If you really need exact values (say, you're working with money) use the
NUMERIC type instead.

--
Craig Ringer