How can I fetch first n rows from a TopLink query?

For optimization purpose, I want to fetch first N results in a subquery (I'm getting first N ID values) and in the main query fetch full rows for the ID values in the subquery and order them. What I have now is

// This just adds params to the subquery
Expression managedEmp = generateExpression(p_upravljackaFilter);
ReportQuery subQuery = new ReportQuery(CustomDocument.class,
    managedEmp);

subQuery.addAttribute("m_id");

Expression exp = new ExpressionBuilder().get("m_id").in(subQuery);
ReadAllQuery testQuery = new ReadAllQuery(CustomDocument.class,
    exp);
testQuery.addAscendingOrdering("m_creationDate");

List documentList = (List)getTopLinkTemplate().executeQuery(testQuery, true);

What I'm trying so far is using a user defined function, like this:

ExpressionOperator fetchFirst = new ExpressionOperator();
fetchFirst.setSelector(1);
Vector s = new Vector();
s.addElement("FETCH FIRST 5 ROWS ONLY");
fetchFirst.printsAs(s);
fetchFirst.bePostfix();
fetchFirst.setN开发者_JS百科odeClass(FunctionExpression.class);
ExpressionOperator.initializeOperators();
ExpressionOperator.addOperator(fetchFirst);
expression = expression.and(builder.get("m_datumKreiranja").getFunction(fetchFirst);

This is literally where I stopped so this won't work but it can show you which way I'm heading. Is something like this even possible? I'm using Java 1.4 and toplink 10g.

Really simple, just insert into second line:

managedEmp = managedEmp.postfixSQL("FETCH FIRST 5 ROWS ONLY");

My mistake was in the fact that I tried it like this:

managedEmp.postfixSQL("FETCH FIRST 5 ROWS ONLY");

because I didn't read what postfixSQL does.