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https://issues.apache.org/jira/browse/CALCITE-3956?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17103185#comment-17103185
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Liya Fan commented on CALCITE-3956:
-----------------------------------
Thanks a lot for your feedback.
> I'm dubious about this change: Like Julian, I'm not sure the actual code use
> RelOptCost would change much BUT at the same time, adding a compareCost
> method might let people think that there's a total ordering where there is
> none.
We have explictly stated in the JavaDoc that the {{compareCost}} is a partial
order. In addition, the return type is not int, so the client code cannot use
it mistakenly in scenarios where a total order is expected.
> (The change also breaks API for those projects which implements their own
> RelOptCost)
This is a problem. So we need to consider 1) if there is a beter way; 2) if the
benefits justify the effort.
> Unify comparison logic for RelOptCost
> -------------------------------------
>
> Key: CALCITE-3956
> URL: https://issues.apache.org/jira/browse/CALCITE-3956
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Liya Fan
> Assignee: Liya Fan
> Priority: Major
> Time Spent: 20m
> Remaining Estimate: 0h
>
> Currently, comparisons between RelOptCost objects are based on 3 methods:
> 1. {{boolean isLe(RelOptCost cost)}}
> 2. {{boolean isLt(RelOptCost cost)}}
> 3. {{boolean equals(RelOptCost cost)}}
> The 3 methods used in combination determine the relation between RelOptCost
> objects.
> There are some problems with this implementation:
> 1. Some logic is duplicate in the above methods, making it difficult to
> maintain.
> 2. To determine the relation between RelOptCost objects, we often need to
> call more than one comparison methods, leading to performance overhead.
> 3. Since the logic is spread in multiple methods, it is easy to end up with
> contradictive comparison logic, which will suprise the users. For example,
> the following assertion should hold according to common sense:
> {{if a >=b, then we have a > b or a == b}}
> However, with the current implementation of {{VolcanoCost}}, we can easily
> create instances that violate the above assertion.
> To solve the problems, we want to make {{RelOptCost}} extends the
> {{Comparable<RelOptCost>}}, so the comparison logic is unified in the
> {{compareTo}} method, which solves the above problems.
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