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https://issues.apache.org/jira/browse/CALCITE-3711?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17008681#comment-17008681
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Ruben Q L commented on CALCITE-3711:
------------------------------------
I think we could provide a simple implementation such as:
{code:java}
public abstract class Correlate extends BiRel {
...
@Override public double estimateRowCount(RelMetadataQuery mq) {
return mq.getRowCount(left) * mq.getRowCount(right);
}
{code}
which is very basic, but still better than the current default implementation
(1.0).
[~vladimirsitnikov], I can create a PR with this small change; or maybe we
could consider including this change as part of your big
[PR#1702|https://github.com/apache/calcite/pull/1702], which involves cost
computation and impacts already Correlate.java. What do you think?
> 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
> Priority: Major
>
> 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}} (and by inheritance
> 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 a Correlate
> final double leftRowCount = left.estimateRowCount(mq); // 1.0 if left is
> a 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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