I created the following record batch which has a single column with a type of map<dict, string> where dict is defined as: dict<int8,string>:
When one attempts to send this in a round trip IPC:
- On IpcFormatWriter::Start(): The memo records one entry for field_to_id and id_to_type_ where the dict id = 0.
- On IpcFormatWriter::CollectDictionaries: The memo records a new entry for field_to_id and id_to_type with id=1 and also records in id_to_dictionary_. At this point we have 2 entries with the entry id=0 having no associated dict.
- On IpcFormatWriter;:WriteDictionaries: It writes the dict with entry = 1
- GetSchema eventually gets to the nested dictionary in FieldFromFlatBuffer
- The recovered dict id is 0.
- This adds to the memo the field_to_id and id_to_type with id = 0
- My round trip code calls "ReadAll".
- RecordBatchStreamReaderImpl::ReadNext attempts to load the initial dicts
- It recovers id = 1
- The process aborts because id = 1 is not in the memo: dictionary_memo->GetDictionaryType(id, &value_type)
A similar example with a dict inside a "struct" worked fine and only used dict id = 0. So it looks like something wrong when gathering the schema for the map. Unless I did not construct the map correctly?