The current StoredFieldsFormat are implemented with the assumption that only one type of StoredfieldsFormat is used by the index.
We would like to be able to configure a StoredFieldsFormat per field, similarly to the PostingsFormat.
There is a few issues that need to be solved for allowing that:
1) allowing to configure a segment suffix to the StoredFieldsFormat
2) implement SPI interface in StoredFieldsFormat
3) create a PerFieldStoredFieldsFormat
We are proposing to start first with 1) by modifying the signature of StoredFieldsFormat#fieldsReader and StoredFieldsFormat#fieldsWriter so that they use SegmentReadState and SegmentWriteState instead of the current set of parameters.
Let us know what you think about this idea. If this is of interest, we can contribute with a first path for 1).