presto-main/src/main/java/io/prestosql/operator/index/StreamingIndexedData.java/StreamingIndexedData/extractNonEmptyPage
class StreamingIndexedData:    private static Page extractNonEmptyPage(PageBuffer pageBuffer)
    {
        Page page = pageBuffer.poll();
        while (page != null && page.getPositionCount() == 0) {
            page = pageBuffer.poll();
        }
        return page;
    }