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https://issues.apache.org/jira/browse/OAK-2725?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14483334#comment-14483334
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Florin Iordache commented on OAK-2725:
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cc: [~tmueller]
> Wrong indexed query estimates exceed more than double the actual index entries
> ------------------------------------------------------------------------------
>
> Key: OAK-2725
> URL: https://issues.apache.org/jira/browse/OAK-2725
> Project: Jackrabbit Oak
> Issue Type: Bug
> Components: query
> Affects Versions: 1.1.8
> Reporter: Florin Iordache
> Priority: Critical
> Fix For: 1.2
>
> Attachments: OAK-2725-test.patch
>
>
> The {{ApproximateCounter.adjustCountSync}} public method that is used by the
> indexing engine will sometimes produce very unrealistic cost estimates.
> The problem is that it can produce an estimated cost that exceeds the
> estimated cost of the full traversal query, thus causing the index to be
> bypassed altogether, resulting in a full traversal rather than the use of the
> existing index.
> Problem resides in the way the property counts are updated:
> * The count property update goes through if two randoms are not zero:
> random(100) and random({1, 2, 4, 8, 16, ...}).
> * Same static pseudo random generator for all invocations.
> Even if #1 might seem improbable, it is statistically possible to reach a
> very high count with only a handful of invocations.
> In practice I've found that running 100 tests with 1000 invocations if the
> adjustCountSync method will yield costs exceeding value 2000 in 4-10% of the
> tests. Attaching a patch for {{ApproximateCounterTest}} with this test case.
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