presto-orc/src/main/java/io/prestosql/orc/writer/DictionaryBuilder.java/DictionaryBuilder/clear
class DictionaryBuilder:    public void clear()
    {
        containsNullElement = false;
        blockPositionByHash.fill(EMPTY_SLOT);
        elementBlock = elementBlock.newBlockBuilderLike(null);
        // first position is always null
        elementBlock.appendNull();
    }