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

["Bohdan Kazydub (JIRA)" <ji...@apache.org>]

Bohdan Kazydub created DRILL-7038:
-------------------------------------

             Summary: 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
             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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