[CALCITE-938] More accurate rowCount for Aggregate applied to already unique keys

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 1.5.0
• Assignee: Wei Xue
• Reporter: Wei Xue
• Created: 26/Oct/15 18:35
• Updated: 27/Feb/24 22:24
• Resolved: 26/Oct/15 23:19

Description

If columns in "select distinct" are already distinct, there can be two sets of equivalent rel before and after AggregateRemoveRule.

agg
 |                  input
input
10.0                100.0

Based on the default implementation of rel metadata, the rowCount of the "before" rel is only 1/10 of that of the "after" rel, but meanwhile the "after" rel is definitely cheaper. So the Volcano planner would most likely either fail to pick the cheapest one or have an inconsistent state due to CALCITE-830.

An example (based EnumerableRel cost model):
The plan for

select empno, d.deptno
from "scott".emp
join (select distinct deptno from "scott".dept) d
using (deptno);

would be

EnumerableCalc(expr#0..2=[{inputs}], EMPNO=[$t1], DEPTNO=[$t0])
  EnumerableJoin(condition=[=($0, $2)], joinType=[inner])
    EnumerableAggregate(group=[$0])
      EnumerableTableScan(table=[[scott, DEPT]])
    EnumerableCalc(expr#0..7=[{inputs}], EMPNO=[$t0], DEPTNO=[$t7])
      EnumerableTableScan(table=[[scott, EMP]])

, while it should be

EnumerableCalc(expr#0..2=[{inputs}], EMPNO=[$t1], DEPTNO=[$t0])
  EnumerableJoin(condition=[=($0, $2)], joinType=[inner])
    EnumerableCalc(expr#0..2=[{inputs}], DEPTNO=[$t0])
      EnumerableTableScan(table=[[scott, DEPT]])
    EnumerableCalc(expr#0..7=[{inputs}], EMPNO=[$t0], DEPTNO=[$t7])
      EnumerableTableScan(table=[[scott, EMP]])