[CALCITE-1288] Avoid doing the same join twice if count(distinct) exists

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.9.0
• Assignee: Gautam Parai
• Reporter: Gautam Parai
• Created: 10/Jun/16 23:45
• Updated: 27/Feb/24 22:23
• Resolved: 11/Sep/16 14:40

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, this transformation will be useful when systems don't support grouping sets and instead rely on the join-based plans (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

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]])

Issue Links

• blocks: CALCITE-1356 Release Calcite 1.9.0