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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 Tue, 02 Dec 2014 15:07:13 GMT

    [ https://issues.apache.org/jira/browse/OAK-1907?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14231580#comment-14231580

Thomas Mueller commented on OAK-1907:

For property indexes, usage of the approximate count estimation in a property index can be
disabled by setting the entryCount. If negative, the old mechanism is used (simply counting
the nodes). If another value is used (0 or higher), then the entry count is used as before.
So, to use the approximation, the entryCount property needs to be removed. Existing index
data will not have the correct estimation. To update the statistics, the index could be re-built.
Otherwise, either the old mechanism is used (until there around 100 entries are added), or
a relatively low count is used (100 for example), but this is not different than without the
approximation, so I don't expect that this will result in problems.

For the traversal index, the estimation resolution is relatively low (1000 / 10000). The estimated
count is at least 1000, so in most cases a property index will be used if available (except
for property indexes that have many entries). When traversing only the direct child nodes,
the estimated count divided by 10 is used.

> 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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