presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/RaptorPageSink.java/PartitionedPageWriter/getPageBuffers
class PartitionedPageWriter:        @Override
        public List<PageBuffer> getPageBuffers()
        {
            ImmutableList.Builder<PageBuffer> list = ImmutableList.builder();
            for (PageStore store : pageStores.values()) {
                store.flushToPageBuffer();
                store.getPageBuffer().flush();
                list.add(store.getPageBuffer());
            }
            return list.build();
        }