presto-main/src/main/java/io/prestosql/operator/MultiChannelGroupByHash.java/MultiChannelGroupByHash/createPageWithExtractedDictionary
class MultiChannelGroupByHash:    private Page createPageWithExtractedDictionary(Page page)
    {
        Block[] blocks = new Block[page.getChannelCount()];
        Block dictionary = ((DictionaryBlock) page.getBlock(channels[0])).getDictionary();

        // extract data dictionary
        blocks[channels[0]] = dictionary;

        // extract hash dictionary
        if (inputHashChannel.isPresent()) {
            blocks[inputHashChannel.get()] = ((DictionaryBlock) page.getBlock(inputHashChannel.get())).getDictionary();
        }

        return new Page(dictionary.getPositionCount(), blocks);
    }