presto-main/src/main/java/io/prestosql/operator/project/PageProcessor.java/DictionarySourceIdFunction/apply
class DictionarySourceIdFunction:        @Override
        public DictionaryId apply(DictionaryBlock block)
        {
            return dictionarySourceIds.computeIfAbsent(block.getDictionarySourceId(), ignored -> randomDictionaryId());
        }