[jira] [Resolved] (SPARK-22736) Consider caching decoded dictionaries in VectorizedColumnReader

["Hyukjin Kwon (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SPARK-22736?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Hyukjin Kwon resolved SPARK-22736.
----------------------------------
    Resolution: Incomplete

> Consider caching decoded dictionaries in VectorizedColumnReader
> ---------------------------------------------------------------
>
>                 Key: SPARK-22736
>                 URL: https://issues.apache.org/jira/browse/SPARK-22736
>             Project: Spark
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 2.2.1
>            Reporter: Henry Robinson
>            Priority: Major
>              Labels: bulk-closed
>
> {{VectorizedColumnReader.decodeDictionaryIds()}} calls {{dictionary.decodeToX}} for every dictionary ID encountered in a dict-encoded Parquet page.
> The whole idea of dictionary encoding is that a) values are repeated in a page and b) the dictionary only contains values that are in a page. So we should be able to save some decoding cost by decoding the entire dictionary page once, at the cost of using some memory (but theoretically we could discard the encoded dictionary, I think), and using the decoded dictionary to populate rows. 
> This is particularly true for TIMESTAMP data, which after SPARK-12297, might have a timezone conversion as part of its decoding step.



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