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https://issues.apache.org/jira/browse/DRILL-7038?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16808416#comment-16808416
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Bohdan Kazydub commented on DRILL-7038:
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Hi, [~bbevens]. I think it's OK, but I think it is needed to specify that
additionally for {{DISTINCT}} or {{GROUP BY}} operation the query has to query
({{SELECT}}) partition columns (dir0, dir1,..., dirN) only.
> 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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