How to handle delta dictionaries and finish_preserve_values with nested builders

[adamreichold]

Which part is this question about
How to use the API of arrow_ipc and arrow_array for delta dictionaries when nested builders are involved.

Describe your question
I have implemented delta dictionaries based on the example at https://docs.rs/arrow-ipc/latest/arrow_ipc/writer/struct.StreamWriter.html#example---efficient-delta-dictionaries and this works for top-level columns in my schema by calling column.finish_preserve_values() instead of column.finish() when collecting the arrays for the next RecordBatch.

However, I also have nested builders of lists of structs which contain dictionary fields. The type of the top-level build is then ListBuilder<Box<dyn ArrayBuilder>> which does not have a finish_preserve_values method.

How can I produce the top-level array while instructing the StringDictionaryBuilder hidden inside the StructBuilder inside the ListBuilder to preserve its values?

[adamreichold]

As an aside that might be related, I wonder if multiple arrays can share a dictionary, i.e. some of my top-level columns and nested columns have values from the same small set which would benefit from this.

[adamreichold]

It is not really a minified example, but if it helps, this is my current code for building record batches based on our internal document-oriented schema.