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

["Liya Fan (Jira)" <ji...@apache.org>]

Liya Fan created CALCITE-3956:
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             Summary: 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


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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