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https://issues.apache.org/jira/browse/JCR-1213?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12542078
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Marcel Reutegger commented on JCR-1213:
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Just wanted to let you know that what you described is exactly how I imagined a 
solution. And the same concerns about the reader lookup crossed my mind... But 
let's see if it even matters.

Looking forward to seeing your patch.

> UUIDDocId cache does not work properly because of weakReferences in 
> combination with new instance for combined indexreader 
> ---------------------------------------------------------------------------------------------------------------------------
>
>                 Key: JCR-1213
>                 URL: https://issues.apache.org/jira/browse/JCR-1213
>             Project: Jackrabbit
>          Issue Type: Improvement
>          Components: query
>    Affects Versions: 1.3.3
>            Reporter: Ard Schrijvers
>             Fix For: 1.4
>
>
> Queries that use ChildAxisQuery or DescendantSelfAxisQuery make use of 
> getParent() functions to know wether the parents are correct and if the 
> result is allowed. The getParent() is called recursively for every hit, and 
> can become very expensive. Hence, in DocId.UUIDDocId, the parents are cached. 
> Currently,  docId.UUIDDocId's are cached by having a WeakRefence to the 
> CombinedIndexReader, but, this CombinedIndexReader is recreated all the time, 
> implying that a gc() is allowed to remove the 'expensive' cache.
> A much better solution is to not have a weakReference to the 
> CombinedIndexReader, but to a reference of each indexreader segment. This 
> means, that in getParent(int n) in SearchIndex the return 
> return id.getDocumentNumber(this) needs to be replaced by return 
> id.getDocumentNumber(subReaders[i]); and something similar in 
> CachingMultiReader. 
> That is all. Obviously, when a node/property is added/removed/changed, some 
> parts of the cached DocId.UUIDDocId will be invalid, but mainly small indexes 
> are updated frequently, which obviously are less expensive to recompute.

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