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https://issues.apache.org/jira/browse/DRILL-4771?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15497847#comment-15497847
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Gautam Kumar Parai commented on DRILL-4771:
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I have created the pull request https://github.com/apache/drill/pull/588. 
[~amansinha100] Can you please review the PR? Thanks!

> Drill should avoid doing the same join twice if count(distinct) exists
> ----------------------------------------------------------------------
>
>                 Key: DRILL-4771
>                 URL: https://issues.apache.org/jira/browse/DRILL-4771
>             Project: Apache Drill
>          Issue Type: Improvement
>    Affects Versions: 1.2.0
>            Reporter: Gautam Kumar Parai
>            Assignee: Gautam Kumar Parai
>
> When the query has one distinct aggregate and one or more non-distinct 
> aggregates, the join instance need not produce the join-based plan. We can 
> generate multi-phase aggregates. Another approach would be to use grouping 
> sets. However, Drill is unable to support grouping sets and instead relies on 
> the join-based plan (see the plan below)
> {code}
> select emp.empno, count(*), avg(distinct dept.deptno) 
> from sales.emp emp inner join sales.dept dept 
> on emp.deptno = dept.deptno 
> group by emp.empno
> LogicalProject(EMPNO=[$0], EXPR$1=[$1], EXPR$2=[$3])
>   LogicalJoin(condition=[IS NOT DISTINCT FROM($0, $2)], joinType=[inner])
>     LogicalAggregate(group=[{0}], EXPR$1=[COUNT()])
>       LogicalProject(EMPNO=[$0], DEPTNO0=[$9])
>         LogicalJoin(condition=[=($7, $9)], joinType=[inner])
>           LogicalTableScan(table=[[CATALOG, SALES, EMP]])
>           LogicalTableScan(table=[[CATALOG, SALES, DEPT]])
>     LogicalAggregate(group=[{0}], EXPR$2=[AVG($1)])
>       LogicalAggregate(group=[{0, 1}])
>         LogicalProject(EMPNO=[$0], DEPTNO0=[$9])
>           LogicalJoin(condition=[=($7, $9)], joinType=[inner])
>             LogicalTableScan(table=[[CATALOG, SALES, EMP]])
>             LogicalTableScan(table=[[CATALOG, SALES, DEPT]])
> {code}
> The more efficient form should look like this
> {code}
> select emp.empno, count(*), avg(distinct dept.deptno) 
> from sales.emp emp inner join sales.dept dept 
> on emp.deptno = dept.deptno 
> group by emp.empno
> LogicalAggregate(group=[{0}], EXPR$1=[SUM($2)], EXPR$2=[AVG($1)])
>   LogicalAggregate(group=[{0, 1}], EXPR$1=[COUNT()])
>     LogicalProject(EMPNO=[$0], DEPTNO0=[$9])
>       LogicalJoin(condition=[=($7, $9)], joinType=[inner])
>         LogicalTableScan(table=[[CATALOG, SALES, EMP]])
>         LogicalTableScan(table=[[CATALOG, SALES, DEPT]])
> {code}



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