Tim Underwood created LUCENE-8557:
-------------------------------------
Summary: 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
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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