[jira] [Created] (ARROW-17900) [Python] combine_chunks on DictionaryArray appears to be broken

["Jared Weston (Jira)" <ji...@apache.org>]

Jared Weston created ARROW-17900:
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             Summary: [Python] combine_chunks on DictionaryArray appears to be broken
                 Key: ARROW-17900
                 URL: https://issues.apache.org/jira/browse/ARROW-17900
             Project: Apache Arrow
          Issue Type: Bug
          Components: Python
            Reporter: Jared Weston
         Attachments: category_counts.py, test.parquet

Recently upgraded from pyarrow 4.0.1 to 9.0.0 and there appears to be a bug when combining the chunks of a dictionary with multiple row groups.  The dictionary is a stringarray of categories.

It is worth noting here that each category is not present in each chunk. To me, the issue appears to be that the category indices per chunk appear to be incorrect when a category is missing from a chunk when they are combined together. I assume this as counts for the categories of a lower index (0, 1) appear to be more frequent in the bugged version compared to the working version, and the counts of the lower indices (2, 3, 4) are lower.

 

The difference can be easily noted when running a value count. For example;

!two.png!

A workaround for now is to read directly as a string array, and then encode this as a dictionary. This isn't the best however due to speed and memory concerns.

!one.png!

 

Attached is my table (I did not create this - so excuse the data / uuid style column names) and a script to see the difference. Please run this with pyarrow 4.0.1 and pyarrow 9.0.0 to see the difference in output.



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