presto-main/src/main/java/io/prestosql/operator/TableScanOperator.java/TableScanOperator/isFinished
class TableScanOperator:    @Override
    public boolean isFinished()
    {
        if (!finished) {
            finished = (source != null) && source.isFinished();
            if (source != null) {
                systemMemoryContext.setBytes(source.getSystemMemoryUsage());
            }
        }

        return finished;
    }