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https://issues.apache.org/jira/browse/LUCENE-7579?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15760882#comment-15760882
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Ferenczi Jim commented on LUCENE-7579:
--------------------------------------
I pushed another commit that removes the specialized API for sorting a
StoredFieldsWriter. This is now done directly in the StoredFieldsConsumer with
a custom CopyVisitor (copied from MergeVisitor).
I've also added some asserts that check if unsorted segments were built from
version prior to Lucene 7.0. We'll need to change the assert when this gets
backported to 6.x. I could not add the assert on maybeSortReaders because
IndexWriter.addIndexes uses the merge to add indices that could be unsorted. I
don't know if this should be allowed or not but we can revisit this later.
Other than that I think it's ready !
> Sorting on flushed segment
> --------------------------
>
> Key: LUCENE-7579
> URL: https://issues.apache.org/jira/browse/LUCENE-7579
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Ferenczi Jim
>
> Today flushed segments built by an index writer with an index sort specified
> are not sorted. The merge is responsible of sorting these segments
> potentially with others that are already sorted (resulted from another
> merge).
> I'd like to investigate the cost of sorting the segment directly during the
> flush. This could make the merge faster since they are some cheap
> optimizations that can be done only if all segments to be merged are sorted.
> For instance the merge of the points could use the bulk merge instead of
> rebuilding the points from scratch.
> I made a small prototype which sort the segment on flush here:
> https://github.com/apache/lucene-solr/compare/master...jimczi:flush_sort
> The idea is simple, for points, norms, docvalues and terms I use the
> SortingLeafReader implementation to translate the values that we have in RAM
> in a sorted enumeration for the writers.
> For stored fields I use a two pass scheme where the documents are first
> written to disk unsorted and then copied to another file with the correct
> sorting. I use the same stored field format for the two steps and just remove
> the file produced by the first pass at the end of the process.
> This prototype has no implementation for index sorting that use term vectors
> yet. I'll add this later if the tests are good enough.
> Speaking of testing, I tried this branch on [~mikemccand] benchmark scripts
> and compared master with index sorting against my branch with index sorting
> on flush. I tried with sparsetaxis and wikipedia and the first results are
> weird. When I use the SerialScheduler and only one thread to write the docs,
> index sorting on flush is slower. But when I use two threads the sorting on
> flush is much faster even with the SerialScheduler. I'll continue to run the
> tests in order to be able to share something more meaningful.
> The tests are passing except one about concurrent DV updates. I don't know
> this part at all so I did not fix the test yet. I don't even know if we can
> make it work with index sorting ;).
> [~mikemccand] I would love to have your feedback about the prototype. Could
> you please take a look ? I am sure there are plenty of bugs, ... but I think
> it's a good start to evaluate the feasibility of this feature.
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