[jira] [Comment Edited] (ARROW-3772) [C++] Read Parquet dictionary encoded ColumnChunks directly into an Arrow DictionaryArray

["Wes McKinney (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-3772?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16888054#comment-16888054 ] 

Wes McKinney edited comment on ARROW-3772 at 7/18/19 2:59 PM:
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At least after ARROW-3144 we have broken the constraint of a constant dictionary across arrays. Having a mix of dictionary-encoded and non-dictionary-encoded arrays is interesting, but regardless there's a lot of refactoring to do in the Parquet library to expose these details


was (Author: wesmckinn):
At least after ARROW-3144 we have broken the constraint of a constant dictionary across arrays. Having a mix of dictionary-encoded and non-dictionary-encoded arrays is interesting, but regardless there's a lot of refactoring to do in the Parquet library to expose this 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
>             Fix For: 1.0.0
>
>
> 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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