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  1. Apache Drill
  2. DRILL-4771

Drill should avoid doing the same join twice if count(distinct) exists

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    Details

    • Type: Improvement
    • Status: Resolved
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: 1.2.0
    • Fix Version/s: 1.9.0
    • Component/s: None
    • Labels:
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      Description

      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)

      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]])
      

      The more efficient form should look like this

      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]])
      
      

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            • Assignee:
              gparai Gautam Parai
              Reporter:
              gparai Gautam Parai
              Reviewer:
              Khurram Faraaz
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              Dates

              • Created:
                Updated:
                Resolved: