presto-main/src/main/java/io/prestosql/operator/MultiChannelGroupByHash.java/MultiChannelGroupByHash/startNewPage
class MultiChannelGroupByHash:    private void startNewPage()
    {
        if (currentPageBuilder != null) {
            completedPagesMemorySize += currentPageBuilder.getRetainedSizeInBytes();
            currentPageBuilder = currentPageBuilder.newPageBuilderLike();
        }
        else {
            currentPageBuilder = new PageBuilder(types);
        }

        for (int i = 0; i < types.size(); i++) {
            channelBuilders.get(i).add(currentPageBuilder.getBlockBuilder(i));
        }
    }