[jira] [Commented] (HIVE-25827) Parquet file footer is read multiple times, when multiple splits are created in same file

["Ádám Szita (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-25827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17554048#comment-17554048 ] 

Ádám Szita commented on HIVE-25827:
-----------------------------------

We don't need multiple splits of the same file for this to happen. Every file is opened 2 times with the vectorized reader. And yes, e.g. in LLAP's case that's two different streams as in that scenario there's a custom logic to load the metadata part of the file into metadata cache. If there's a cache hit, 1 of the reads would be served from cache, but the other occasion reads from file in the dumb way nevertheless.

> Parquet file footer is read multiple times, when multiple splits are created in same file
> -----------------------------------------------------------------------------------------
>
>                 Key: HIVE-25827
>                 URL: https://issues.apache.org/jira/browse/HIVE-25827
>             Project: Hive
>          Issue Type: Improvement
>            Reporter: Rajesh Balamohan
>            Assignee: Ádám Szita
>            Priority: Major
>              Labels: performance
>         Attachments: image-2021-12-21-03-19-38-577.png
>
>
> With large files, it is possible that multiple splits are created in the same file. With current codebase, "ParquetRecordReaderBase" ends up reading file footer for each split. 
> It can be optimized not to read footer information multiple times for the same file.
>  
> [https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/io/parquet/vector/VectorizedParquetRecordReader.java#L160]
>  
> [https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/io/parquet/ParquetRecordReaderBase.java#L91]
>  
>  
> !image-2021-12-21-03-19-38-577.png|width=1363,height=1256!
>  



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