[jira] [Created] (CALCITE-3711) Correlate should override estimateRowCount

["Ruben Q L (Jira)" <ji...@apache.org>]

Ruben Q L created CALCITE-3711:
----------------------------------

             Summary: Correlate should override estimateRowCount
                 Key: CALCITE-3711
                 URL: https://issues.apache.org/jira/browse/CALCITE-3711
             Project: Calcite
          Issue Type: Improvement
    Affects Versions: 1.21.0
            Reporter: Ruben Q L


Currently, Correlate inherits the {{estimateRowCount}} implementation from AbstractRelNode:
{code:java}
public double estimateRowCount(RelMetadataQuery mq) {
  return 1.0;
}
{code}
which gives a "default" (but unrealistic) row count computation.
This has several issues:
- This value may be used by Correlate#computeSelfCost (i.e. by LogicalCorrelate / EnumerableCorrelate):
{code:java}
@Override public RelOptCost computeSelfCost(final RelOptPlanner planner, final RelMetadataQuery mq) {
  double rowCount = mq.getRowCount(this);  // by default: estimateRowCount, i.e. 1.0
  ...
{code}
- As the current state, this value will definitely by used by several join algorithms in their cost computation if their left/right child is a Correlate:
{code:java}
public class EnumerableHashJoin extends Join implements EnumerableRel {
...
  @Override public RelOptCost computeSelfCost(RelOptPlanner planner, RelMetadataQuery mq) {
  ...
  final double rightRowCount = right.estimateRowCount(mq); // 1.0 if right is Correlate
  final double leftRowCount = left.estimateRowCount(mq); // 1.0 if left is Correlate
  ... 
{code}

Even though cost computation is pluggable (which would help solving issues like the first one, but not issues like the second one), IMHO we should provide a more realistic default {{Correlate#estimateRowCount}} computation.



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