[jira] [Commented] (DRILL-7038) Queries on partitioned columns scan the entire datasets

["Bridget Bevens (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/DRILL-7038?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16803526#comment-16803526 ] 

Bridget Bevens commented on DRILL-7038:
---------------------------------------

Hi [~KazydubB],

I'm planning to add the following information to this page: https://drill.apache.org/docs/querying-directories/ 
Can you please review and let me know if the information is accurate?

Starting in Drill 1.16, if a query on a file system table references partitioned columns only and also has a DISTINCT or GROUP BY operation, Drill only reads the data partition(s) referenced. For example, if a table, table1, is partitioned into the following directories:
{{/table1/2016
/table1/2017
/table1/2018}}

And you run either of the following queries against table1:

{{select distinct dir0 from dfs.`/table`;
select dir0 from dfs.`/table` group by dir0;}}

Drill only scans dir0 (/table1/2016) instead of scanning all three of the directories.  

Thanks,
Bridget


> Queries on partitioned columns scan the entire datasets
> -------------------------------------------------------
>
>                 Key: DRILL-7038
>                 URL: https://issues.apache.org/jira/browse/DRILL-7038
>             Project: Apache Drill
>          Issue Type: Improvement
>            Reporter: Bohdan Kazydub
>            Assignee: Bohdan Kazydub
>            Priority: Major
>              Labels: doc-impacting, ready-to-commit
>             Fix For: 1.16.0
>
>
> For tables with hive-style partitions like
> {code}
> /table/2018/Q1
> /table/2018/Q2
> /table/2019/Q1
> etc.
> {code}
> if any of the following queries is run:
> {code}
> select distinct dir0 from dfs.`/table`
> {code}
> {code}
> select dir0 from dfs.`/table` group by dir0
> {code}
> it will actually scan every single record in the table rather than just getting a list of directories at the dir0 level. This applies even when cached metadata is available. This is a big penalty especially as the datasets grow.
> To avoid such situations, a logical prune rule can be used to collect partition columns (`dir0`), either from metadata cache (if available) or group scan, and drop unnecessary files from being read. The rule will be applied on following conditions:
> 1) all queried columns are partitoin columns, and
> 2) either {{DISTINCT}} or {{GROUP BY}} operations are performed.



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