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Posted to issues@arrow.apache.org by "Wes McKinney (JIRA)" <ji...@apache.org> on 2019/07/26 23:55:00 UTC
[jira] [Resolved] (ARROW-3772) [C++] Read Parquet dictionary
encoded ColumnChunks directly into an Arrow DictionaryArray
[ https://issues.apache.org/jira/browse/ARROW-3772?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Wes McKinney resolved ARROW-3772.
---------------------------------
Resolution: Fixed
Issue resolved by pull request 4949
[https://github.com/apache/arrow/pull/4949]
> [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: 1.0.0
>
> Time Spent: 1h 20m
> 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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