presto-spi/src/main/java/io/prestosql/spi/block/DictionaryBlockEncoding.java/DictionaryBlockEncoding/writeBlock
class DictionaryBlockEncoding:    @Override
    public void writeBlock(BlockEncodingSerde blockEncodingSerde, SliceOutput sliceOutput, Block block)
    {
        // The down casts here are safe because it is the block itself the provides this encoding implementation.
        DictionaryBlock dictionaryBlock = (DictionaryBlock) block;

        dictionaryBlock = dictionaryBlock.compact();

        // positionCount
        int positionCount = dictionaryBlock.getPositionCount();
        sliceOutput.appendInt(positionCount);

        // dictionary
        Block dictionary = dictionaryBlock.getDictionary();
        blockEncodingSerde.writeBlock(sliceOutput, dictionary);

        // ids
        sliceOutput.writeBytes(dictionaryBlock.getIds());

        // instance id
        sliceOutput.appendLong(dictionaryBlock.getDictionarySourceId().getMostSignificantBits());
        sliceOutput.appendLong(dictionaryBlock.getDictionarySourceId().getLeastSignificantBits());
        sliceOutput.appendLong(dictionaryBlock.getDictionarySourceId().getSequenceId());
    }