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

Question

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.setNodeClass(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.

Answer 1

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.