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https://issues.apache.org/jira/browse/CALCITE-4049?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17134589#comment-17134589
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Xiening Dai commented on CALCITE-4049:
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After looking it deeper, I feel the shorted path might not needed at all.
Right now, the getShortestPath() is used only by
ConventionTraitDef.canConvert(). [1] But actually in
ConventionTraitDef.convert(), it doesn't use the shortest path for conversion,
instead it gets all possible paths and enumerate them one by one. [2] So it
looks to me that the shortest path is not used at all. And even though the
comment of Graph#getPaths() says it returns shortest path first, but actually
it uses a DFS to find the path, and doesn't grantee shortest first today. So I
believe the getShortedPath() can be removed.
[1]
https://github.com/apache/calcite/blob/52a57078ba081b24b9d086ed363c715485d1a519/core/src/main/java/org/apache/calcite/plan/ConventionTraitDef.java#L200
[2]
https://github.com/apache/calcite/blob/52a57078ba081b24b9d086ed363c715485d1a519/core/src/main/java/org/apache/calcite/plan/ConventionTraitDef.java#L139
> Reduce the time complexity of getting shortest distances
> --------------------------------------------------------
>
> Key: CALCITE-4049
> URL: https://issues.apache.org/jira/browse/CALCITE-4049
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Liya Fan
> Assignee: Liya Fan
> Priority: Major
> Fix For: 1.24.0
>
> Time Spent: 1h 20m
> Remaining Estimate: 0h
>
> Currently, we have {{Graphs#makeImmutable}} to compute the shortest paths
> between all pairs of nodes. For many scenarios, however, we do not need the
> exact paths between nodes. Instead, we are only interested in the lengths of
> the shortest paths.
> To get the path length, we need to get the shortest path first, which is
> returned as a {{List}}, then we call the {{List#size()}} method. According to
> the current implementation, the returned list is of type {{ConsList}}. The
> time complexity of {{ConsList#size}} is O(p) (p is the number of vertices on
> the path), which is inefficient.
> In this issue, we revise the implementation of {{ConsList#size}} so that it
> takes O(1) time. In addition, we also give a utiltiy to get the shortest
> distances between nodes.
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