Reading Parquet File with Array<Map<String,String>> Column

Question

I'm using Dask to read a Parquet file that was generated by PySpark, and one of the columns is a list of dictionaries (i.e. array<map<string,string>>'). An example of the df would be:

import pandas as pd

df = pd.DataFrame.from_records([ 
    (1, [{'job_id': 1, 'started': '2019-07-04'}, {'job_id': 2, 'started': '2019-05-04'}], 100), 
    (5, [{'job_id': 3, 'started': '2015-06-04'}, {'job_id': 9, 'started': '2019-02-02'}], 540)], 
    columns=['uid', 'job_history', 'latency'] 
) 

The when using engine='fastparquet, Dask reads all other columns fine but returns a column of Nones for the column with the complex type. When I set engine='pyarrow', I get the following exception:

ArrowNotImplementedError: lists with structs are not supported.

A lot of googling has made it clear that reading a column with a Nested Array just isn't really supported right now, and I'm not totally sure what the best way to handle this is. I figure my options are:

• Some how tell dask/fastparquet to parse the column using the standard json library. The schema is simple and that would do the job if possible
• See if I can possibily re-run the Spark job that generated the output and save it as something else, though this almost isn't an acceptable solution since my company uses parquet everywhere
• Turn the keys of the map into columns and break the data up across several columns with dtype list and note that the data across these columns are related/map to each other by index (e.g. the elements in idx 0 across these keys/columns all came from the same source). This would work, but frankly, breaks my heart :(

I'd love to hear how others have navigated around this limitation. My company uses nested arrays in their parquest frequently, and I'd hate to have to let go of using Dask because of this.

Answer 1

It would be fairer to say that pandas does not support non-simple types very well (currently). It may be the case that pyarrow will, without conversion to pandas, and that as some future point, pandas will use these arrow structures directly.

Indeed, the most direct method that I can think for you to use, is to rewrite the columns as B/JSON-encoded text, and then load with fastparquet, specifying to load using B/JSON. You should get lists of dicts in the column, but the performance will be slow.

Note that the old project oamap and its successor awkward provides a way to iterate and aggregate over nested list/map/struct trees using python syntax, but compiled with Numba, such that you never need to instantiate the intermediate python objects. They were not designed for parquet, but had parquet compatibility, so might just be useful to you.