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  1. Lucene - Core
  2. LUCENE-8962

Can we merge small segments during refresh, for faster searching?


    • Improvement
    • Status: Closed
    • Major
    • Resolution: Fixed
    • None
    • 9.0, 8.6, 8.7
    • core/index
    • None
    • New


      Two improvements were added: 8.6 has merge-on-commit (by Froh et. all), 8.7 has merge-on-refresh (by Simon).  See MergePolicy.findFullFlushMerges

      The original description follows:

      With near-real-time search we ask IndexWriter to write all in-memory segments to disk and open an IndexReader to search them, and this is typically a quick operation.

      However, when you use many threads for concurrent indexing, IndexWriter will accumulate write many small segments during refresh and this then adds search-time cost as searching must visit all of these tiny segments.

      The merge policy would normally quickly coalesce these small segments if given a little time ... so, could we somehow improve {{IndexWriter'}}s refresh to optionally kick off merge policy to merge segments below some threshold before opening the near-real-time reader?  It'd be a bit tricky because while we are waiting for merges, indexing may continue, and new segments may be flushed, but those new segments shouldn't be included in the point-in-time segments returned by refresh ...

      One could almost do this on top of Lucene today, with a custom merge policy, and some hackity logic to have the merge policy target small segments just written by refresh, but it's tricky to then open a near-real-time reader, excluding newly flushed but including newly merged segments since the refresh originally finished ...

      I'm not yet sure how best to solve this, so I wanted to open an issue for discussion!


        1. LUCENE-8962_demo.png
          24 kB
          Michael Froh
        2. failed-tests.patch
          8 kB
          Nhat Nguyen
        3. failure_log.txt
          36.16 MB
          Simon Willnauer
        4. test.diff
          3 kB
          Simon Willnauer
        5. image-2021-03-22-10-36-32-201.png
          4 kB
          Michael McCandless

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              Unassigned Unassigned
              mikemccand Michael McCandless
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