presto-main/src/main/java/io/prestosql/operator/NestedLoopJoinOperator.java/NestedLoopJoinOperator/getOutput
class NestedLoopJoinOperator:    @Override
    public Page getOutput()
    {
        // Either probe side or build side is not ready
        if (probePage == null || buildPages == null) {
            return null;
        }

        if (nestedLoopPageBuilder != null && nestedLoopPageBuilder.hasNext()) {
            return nestedLoopPageBuilder.next();
        }

        if (buildPageIterator.hasNext()) {
            nestedLoopPageBuilder = new NestedLoopPageBuilder(probePage, buildPageIterator.next());
            return nestedLoopPageBuilder.next();
        }

        probePage = null;
        return null;
    }