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https://issues.apache.org/jira/browse/ARROW-3772?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17660795#comment-17660795
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Rok Mihevc commented on ARROW-3772:
-----------------------------------
This issue has been migrated to [issue
#20110|https://github.com/apache/arrow/issues/20110] on GitHub. Please see the
[migration documentation|https://github.com/apache/arrow/issues/14542] for
further details.
> [C++] Read Parquet dictionary encoded ColumnChunks directly into an Arrow
> DictionaryArray
> -----------------------------------------------------------------------------------------
>
> Key: ARROW-3772
> URL: https://issues.apache.org/jira/browse/ARROW-3772
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Stav Nir
> Assignee: Wes McKinney
> Priority: Major
> Labels: parquet, pull-request-available
> Fix For: 0.15.0
>
> Time Spent: 2h 50m
> Remaining Estimate: 0h
>
> 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.
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