presto-main/src/main/java/io/prestosql/operator/MultiChannelGroupByHash.java/MultiChannelGroupByHash/getGroupId
class MultiChannelGroupByHash:    private int getGroupId(HashGenerator hashGenerator, Page page, int positionInDictionary)
    {
        if (dictionaryLookBack.isProcessed(positionInDictionary)) {
            return dictionaryLookBack.getGroupId(positionInDictionary);
        }

        int groupId = putIfAbsent(positionInDictionary, page, hashGenerator.hashPosition(positionInDictionary, page));
        dictionaryLookBack.setProcessed(positionInDictionary, groupId);
        return groupId;
    }