Gautam Kumar Parai created CALCITE-1288:
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Summary: Avoid doing the same join twice if count(distinct) exists
Key: CALCITE-1288
URL: https://issues.apache.org/jira/browse/CALCITE-1288
Project: Calcite
Issue Type: Improvement
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.
{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
{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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