[jira] [Commented] (CALCITE-3827) Reduce the time complexity of finding in-edges of a vertex in the graph

["Chunwei Lei (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CALCITE-3827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17045232#comment-17045232 ] 

Chunwei Lei commented on CALCITE-3827:
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Interesting! I would like to review it.

> Reduce the time complexity of finding in-edges of a vertex in the graph
> -----------------------------------------------------------------------
>
>                 Key: CALCITE-3827
>                 URL: https://issues.apache.org/jira/browse/CALCITE-3827
>             Project: Calcite
>          Issue Type: Improvement
>          Components: core
>            Reporter: Liya Fan
>            Priority: Major
>              Labels: pull-request-available
>          Time Spent: 10m
>  Remaining Estimate: 0h
>
> In the current graph implementation, it takes O(1) time to find the out-edges of a vertex, but O(e) time (where e is the total number of edges in the graph) to find the in-edges of a vertex. 
> In some scenarios (e.g. clearing cache in hep planner), this may have severe performance penalty.  To solve this problem, we add a inward edge table, in addition to the existing outward edge table, to the graph. This helps to reduce the time complexity for finding in-edges to O(1). 
> Please note that, there is extra performance overhead for maintaining the in edge table, when adding/deleting vertices to the graph. However, the added overhead only takes O(1) time.
> Finally, it should be noted that, some existing operations can benefit from this improvement (e.g. topological sort). 



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