[jira] [Commented] (OAK-1907) Better cost estimates for traversal, property, and ordered indexes

["Thomas Mueller (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/OAK-1907?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15142461#comment-15142461 ] 

Thomas Mueller commented on OAK-1907:
-------------------------------------

Note: For a 1 million node repository the average error is 8%, for a 10 million nodes repository it is 2%, for a larger repository it is less.

> Better cost estimates for traversal, property, and ordered indexes
> ------------------------------------------------------------------
>
>                 Key: OAK-1907
>                 URL: https://issues.apache.org/jira/browse/OAK-1907
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: query
>    Affects Versions: 1.0, 1.0.1, 1.0.2
>            Reporter: Thomas Mueller
>            Assignee: Thomas Mueller
>             Fix For: 1.1.3
>
>         Attachments: ApproxCount.java, OAK-1907.diff
>
>
> Currently, cost estimates of traversal, property index, and ordered index don't take the number of nodes into account, if there are more than about 100 nodes. This is problematic because in many cases, the wrong index is used (because of incorrect cost estimate).
> To get a better estimate, a very rough estimate on the number of child nodes below a given path is needed. 
> One idea is: when adding a node, if Math.random() < 0.00001, add a hidden, randomly named property (for example called ":count-xyz" where xyz is a uuid, value 100'000) to the parents of that node, so that we know there are probably more than 100'000 nodes below a given path. When removing a node, with the same algorithm add a hidden property (":count-xyz", value -100'000). That should result in a slowdown of less than 0.01%, but should allow us much better cost estimates. Those properties could be consolidated asynchronously if needed.



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