[jira] [Commented] (DRILL-4530) Improve metadata cache performance for queries with single partition

["Deneche A. Hakim (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/DRILL-4530?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15227785#comment-15227785 ] 

Deneche A. Hakim commented on DRILL-4530:
-----------------------------------------

Another alternative is to use protobuf for the metadata cache file. Not only it will use less space and load faster but you can actually only load a subset of the cache in the beginning (e.g. directory list). 
If done right, Drill will be remain backward compatible and use the proper metadata loader depending on the metadata cache format. The only real drawback is that it will require a lot more work than adding a separate .parquet_directories file 

> Improve metadata cache performance for queries with single partition 
> ---------------------------------------------------------------------
>
>                 Key: DRILL-4530
>                 URL: https://issues.apache.org/jira/browse/DRILL-4530
>             Project: Apache Drill
>          Issue Type: Improvement
>          Components: Query Planning & Optimization
>    Affects Versions: 1.6.0
>            Reporter: Aman Sinha
>            Assignee: Aman Sinha
>             Fix For: 1.7.0
>
>
> Consider two types of queries which are run with Parquet metadata caching: 
> {noformat}
> query 1:
> SELECT col FROM  `A/B/C`;
> query 2:
> SELECT col FROM `A` WHERE dir0 = 'B' AND dir1 = 'C';
> {noformat}
> For a certain dataset, the query1 elapsed time is 1 sec whereas query2 elapsed time is 9 sec even though both are accessing the same amount of data.  The user expectation is that they should perform roughly the same.  The main difference comes from reading the bigger metadata cache file at the root level 'A' for query2 and then applying the partitioning filter.  query1 reads a much smaller metadata cache file at the subdirectory level. 



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