presto-main/src/main/java/io/prestosql/operator/PageSourceOperator.java/PageSourceOperator/getOutput
class PageSourceOperator:    @Override
    public Page getOutput()
    {
        Page page = pageSource.getNextPage();
        if (page == null) {
            return null;
        }

        // update operator stats
        long endCompletedBytes = pageSource.getCompletedBytes();
        long endReadTimeNanos = pageSource.getReadTimeNanos();
        operatorContext.recordPhysicalInputWithTiming(endCompletedBytes - completedBytes, page.getPositionCount(), endReadTimeNanos - readTimeNanos);
        operatorContext.recordProcessedInput(page.getSizeInBytes(), page.getPositionCount());
        completedBytes = endCompletedBytes;
        readTimeNanos = endReadTimeNanos;

        // assure the page is in memory before handing to another operator
        page = page.getLoadedPage();

        return page;
    }