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From "Thomas Mueller (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (OAK-1907) Better cost estimates for traversal, property, and ordered indexes
Date Mon, 08 Dec 2014 12:50:13 GMT

    [ https://issues.apache.org/jira/browse/OAK-1907?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14237837#comment-14237837
] 

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

I think the current approximate count implementation for property indexes is not "cluster
safe": it can cause merge conflicts (with a low probability, but it's possible). I need to
work on this.

> 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: 1.2
>
>         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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