presto-main/src/main/java/io/prestosql/operator/SetBuilderOperator.java/SetBuilderOperator/needsInput
class SetBuilderOperator:    @Override
    public boolean needsInput()
    {
        // Since SetBuilderOperator doesn't produce any output, the getOutput()
        // method may never be called. We need to handle any unfinished work
        // before addInput() can be called again.
        return !finished && (unfinishedWork == null || processUnfinishedWork());
    }