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https://issues.apache.org/jira/browse/OAK-11764?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Nuno Santos updated OAK-11764:
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Priority: Minor (was: Major)
> Query planner: when multiple indexes have same cost, planner should choose
> index deterministically
> --------------------------------------------------------------------------------------------------
>
> Key: OAK-11764
> URL: https://issues.apache.org/jira/browse/OAK-11764
> Project: Jackrabbit Oak
> Issue Type: Bug
> Components: indexing
> Reporter: Nuno Santos
> Priority: Minor
> Labels: indexing
>
> The query planner chooses the index with the lowest cost. But if multiple
> indexes evaluate to the same cost, currently the planner is taking the first
> index that is evaluated with the lowest score. This is not deterministic, so
> the same query may be evaluated sometimes by an index and other times by a
> different index. Although the results should be the same regardless of the
> index being used, there can be some cases where the customers are (wrongly)
> relying on the query being evaluated by the same index every time. For
> instance, if the query does not precisely define the order of the results,
> and different indexes will return different orders. This does not violate the
> contract of the query specification, but users may rely on the results being
> always in the same order (for instance, to implement pagination).
> The indexes are evaluated in the class
> [FullTextIndex|[https://github.com/apache/jackrabbit-oak/blob/ff1080185d21bd10c017d8a8eb94236ef99c7e87/oak-search/src/main/java/org/apache/jackrabbit/oak/plugins/index/search/spi/query/FulltextIndex.java#L116-L133],]
> and taken from a Set with all the indexes created in
> [IndexLookup|[https://github.com/apache/jackrabbit-oak/blob/ff1080185d21bd10c017d8a8eb94236ef99c7e87/oak-search/src/main/java/org/apache/jackrabbit/oak/plugins/index/search/IndexLookup.java#L55-L70].]
> This class uses a JDK HashSet to store the indexes. The iteration order of
> this class is changed on purpose from run to run of the JDK. Previously, this
> class was using a Guava HashSet, which iterates always by the order by which
> the elements are added. This
> [PR|https://github.com/apache/jackrabbit-oak/pull/1678] changed the Guava to
> the JDK HashSet.
> We should ensure that the indexes are selected in a deterministic order, even
> if they have the same cost.
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