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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