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https://issues.apache.org/jira/browse/DRILL-4147?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15410511#comment-15410511
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Robert Hou commented on DRILL-4147:
-----------------------------------
Here is a simple test case using lineitem. Lineitem can be small, but it needs
to be created with many parquet files.
alter session set `store.parquet.block-size`=500000;
create table lineitemfiles as select * from lineitem;
create table newlineitemfiles as
with lineitem_cte as
(
select l.l_orderkey, l.l_partkey
from lineitemfiles l limit 1)
(select l.l_orderkey, l.l_partkey
from
lineitemfiles l
inner join
orders o
on l.l_orderkey = o.o_orderkey)
union all
(select l.l_orderkey, l.l_partkey
from lineitem_cte l);
> Union All operator runs in a single fragment
> --------------------------------------------
>
> Key: DRILL-4147
> URL: https://issues.apache.org/jira/browse/DRILL-4147
> Project: Apache Drill
> Issue Type: Bug
> Reporter: amit hadke
> Assignee: Aman Sinha
>
> A User noticed that running select from a single directory is much faster
> than union all on two directories.
> (https://drill.apache.org/blog/2014/12/09/running-sql-queries-on-amazon-s3/#comment-2349732267)
>
> It seems like UNION ALL operator doesn't parallelize sub scans (its using
> SINGLETON for distribution type). Everything is ran in single fragment.
> We may have to use SubsetTransformer in UnionAllPrule.
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