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https://issues.apache.org/jira/browse/LUCENE-8557?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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David Smiley resolved LUCENE-8557.
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Resolution: Fixed
> LeafReader.getFieldInfos should always return the same instance
> ---------------------------------------------------------------
>
> Key: LUCENE-8557
> URL: https://issues.apache.org/jira/browse/LUCENE-8557
> Project: Lucene - Core
> Issue Type: Bug
> Affects Versions: 7.5
> Reporter: Tim Underwood
> Assignee: David Smiley
> Priority: Major
> Attachments: LUCENE-8557.patch
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> Most implementations of the LeafReader cache an instance of FieldInfos which
> is returned in the LeafReader.getFieldInfos() method. There are a few places
> that currently do not and this can cause performance problems.
> The most notable example is the lack of caching in Solr's
> SlowCompositeReaderWrapper which caused unexpected performance slowdowns when
> trying to use Solr's JSON Facets compared to the legacy facets.
> This proposed change is mostly relevant to Solr but touches a few Lucene
> classes. Specifically:
> *1.* Adds a check to TestUtil.checkReader to verify that
> LeafReader.getFieldInfos() returns the same instance:
>
> {code:java}
> // FieldInfos should be cached at the reader and always return the same
> instance
> if (reader.getFieldInfos() != reader.getFieldInfos()) {
> throw new RuntimeException("getFieldInfos() returned different instances for
> class: "+reader.getClass());
> }
> {code}
> I'm not entirely sure this is wanted or needed but adding it uncovered most
> of the other LeafReader implementations that were not caching FieldInfos.
> I'm happy to remove this part of the patch though.
>
> *2.* Adds a FieldInfos.EMPTY that can be used in a handful of places
>
> {code:java}
> public final static FieldInfos EMPTY = new FieldInfos(new FieldInfo[0]);
> {code}
> There are several places in the Lucene/Solr tests that were creating empty
> instances of FieldInfos which were causing the check in #1 to fail. This
> fixes those failures and cleans up the code a bit.
> *3.* Fixes a few LeafReader implementations that were not caching FieldInfos
> Specifically:
> * *MemoryIndex.MemoryIndexReader* - The constructor was already looping over
> the fields so it seemed natural to just create the FieldInfos at that time
> * *SlowCompositeReaderWrapper* - This was the one causing me trouble. I've
> moved the caching of FieldInfos from SolrIndexSearcher to
> SlowCompositeReaderWrapper.
> * *CollapsingQParserPlugin.ReaderWrapper* - getFieldInfos() is immediately
> called twice after this is constructed
> * *ExpandComponent.ReaderWrapper* - getFieldInfos() is immediately called
> twice after this is constructed
>
> *4.* Minor Solr tweak to avoid calling SolrIndexSearcher.getSlowAtomicReader
> in FacetFieldProcessorByHashDV. This change is now optional since
> SlowCompositeReaderWrapper caches FieldInfos.
>
> As suggested by [~dsmiley] this takes the place of SOLR-12878 since it
> touches some Lucene code.
>
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