Uwe Schindler created LUCENE-6501:
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Summary: Flatten subreader structure in ParallelCompositeReader
Key: LUCENE-6501
URL: https://issues.apache.org/jira/browse/LUCENE-6501
Project: Lucene - Core
Issue Type: Improvement
Components: core/index
Affects Versions: 5.2
Reporter: Uwe Schindler
Assignee: Uwe Schindler
Fix For: Trunk, 5.3
The current implementation of ParallelCompositeReader reassembles the whole
subreader structure of the wrapped reader with ParallelLeafReader and
ParallelCompositeReader.
This leads to bugs like described in LUCENE-6500. This reaches back to the time
when this reader was reimplemented for the first time shortly before release of
4.0. Shortly afterwards, we completely changed our search infrastructure to
just call leaves() and working with them. The method getSequentialSubReaders
was made protected, just to be implemented by subclasses (like this one). But
no external code can ever call it. Also the search API just rely on the baseId
in relation to the top-level reader (to correctly present document ids). The
structure is completely unimportant.
This issue will therefore simplify ParallelCompositeReader to just fetch all
LeafReaders and build a flat structure of ParallelLeafReaders from it. This
also has the nice side-effect, that only the parallel leaf readers must be
equally sized, not their structure.
This issue will solve LUCENE-6500 as a side effect. I just opened a new issue
for discussion and to have this listed as "feature" and not bug.
In general, we could also hide the ParallelLeafReader class and make it an
implementation detail. ParallelCompositeReader would be the only entry point ->
because people could pass any IndexReader structure in, a single AtomicReader
would just produce a CompositeReader with one leaf. We could then also rename
it back to ParallelReader (like it was in pre Lucene4).
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