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https://issues.apache.org/jira/browse/ARROW-3408?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16918629#comment-16918629
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Antoine Pitrou commented on ARROW-3408:
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[~wesmckinn] Are chunked dictionary arrays still supposed to have the same
dictionary for all chunks?
> [C++] Add option to CSV reader to dictionary encode individual columns or all
> string / binary columns
> -----------------------------------------------------------------------------------------------------
>
> Key: ARROW-3408
> URL: https://issues.apache.org/jira/browse/ARROW-3408
> Project: Apache Arrow
> Issue Type: New Feature
> Components: C++
> Reporter: Wes McKinney
> Priority: Major
> Labels: csv, dataset
> Fix For: 1.0.0
>
>
> For many datasets, dictionary encoding everything can result in drastically
> lower memory usage and subsequently better performance in doing analytics
> One difficulty of dictionary encoding in multithreaded conversions is that
> ideally you end up with one dictionary at the end. So you have two options:
> * Implement a concurrent hashing scheme -- for low cardinality dictionaries,
> the overhead associated with mutex contention will not be meaningful, for
> high cardinality it can be more of a problem
> * Hash each chunk separately, then normalize at the end
> My guess is that a crude concurrent hash table with a mutex to protect
> mutations and resizes is going to outperform the latter
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