Dictionary data is very common in parquet, in the current implementation parquet-cpp decodes dictionary encoded data always before creating a plain arrow array. This process is wasteful since we could use arrow's DictionaryArray directly and achieve several benefits:
- Smaller memory footprint - both in the decoding process and in the resulting arrow table - especially when the dict values are large
- Better decoding performance - mostly as a result of the first bullet - less memory fetches and less allocations.
I think those benefits could achieve significant improvements in runtime.
My direction for the implementation is to read the indices (through the DictionaryDecoder, after the RLE decoding) and values separately into 2 arrays and create a DictionaryArray using them.
There are some questions to discuss:
- Should this be the default behavior for dictionary encoded data
- Should it be controlled with a parameter in the API
- What should be the policy in case some of the chunks are dictionary encoded and some are not.
I started implementing this but would like to hear your opinions.