Thomas Mueller updated OAK-7379:
    Labels: candidate_oak_1_8  (was: )

> Lucene Index: per-column selectivity, assume 5 unique entries
> -------------------------------------------------------------
>                 Key: OAK-7379
>                 URL: https://issues.apache.org/jira/browse/OAK-7379
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: lucene, query
>            Reporter: Thomas Mueller
>            Assignee: Thomas Mueller
>            Priority: Major
>              Labels: candidate_oak_1_8
>             Fix For: 1.10
> Currently, if a query has a property restriction of the form "property = x", 
> and the property is indexed in a Lucene property index, the estimated cost is 
> the index is the number of documents indexed for that property. This is a 
> very conservative estimate, it means all documents have the same value. So 
> the cost is relatively high for that index.
> In almost all cases, there are many distinct values for a property. Rarely 
> there are few values, or a skewed distribution where one value contains most 
> documents. But in almost all cases there are more than 5 distinct values.
> I think it makes sense to use 5 as the default value. It is still 
> conservative (cost of the index is high), but much better than now.

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