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  1. Calcite
  2. CALCITE-938

More accurate rowCount for Aggregate applied to already unique keys

    Details

    • Type: Improvement
    • Status: Closed
    • Priority: Minor
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 1.5.0
    • Component/s: None
    • Labels:
      None

      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]])
      

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            • Assignee:
              maryannxue Maryann Xue
              Reporter:
              maryannxue Maryann Xue
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              Dates

              • Created:
                Updated:
                Resolved: