[jira] [Commented] (CALCITE-3956) Unify comparison logic for RelOptCost

["Julian Hyde (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CALCITE-3956?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17091680#comment-17091680 ] 

Julian Hyde commented on CALCITE-3956:
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No. The problems arise because RelOptCost is a partial order, not a total order. It is irresponsible and incorrect to make a partial order implement Comparable. 

> 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: 10m
>  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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