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  1. Lucene - Core
  2. LUCENE-3102

Few issues with CachingCollector


    • Bug
    • Status: Closed
    • Minor
    • Resolution: Fixed
    • None
    • 3.2, 4.0-ALPHA
    • core/search
    • None
    • New, Patch Available


      CachingCollector (introduced in LUCENE-1421) has few issues:

      1. Since the wrapped Collector may support out-of-order collection, the document IDs cached may be out-of-order (depends on the Query) and thus replay(Collector) will forward document IDs out-of-order to a Collector that may not support it.
      2. It does not clear cachedScores + cachedSegs upon exceeding RAM limits
      3. I think that instead of comparing curScores to null, in order to determine if scores are requested, we should have a specific boolean - for clarity
      4. This check "if (base + nextLength > maxDocsToCache)" (line 168) can be relaxed? E.g., what if nextLength is, say, 512K, and I cannot satisfy the maxDocsToCache constraint, but if it was 10K I would? Wouldn't we still want to try and cache them?


      • The TODO in line 64 (having Collector specify needsScores()) – why do we need that if CachingCollector ctor already takes a boolean "cacheScores"? I think it's better defined explicitly than implicitly?
      • Let's introduce a factory method for creating a specialized version if scoring is requested / not (i.e., impl the TODO in line 189)
      • I think it's a useful collector, which stands on its own and not specific to grouping. Can we move it to core?
      • How about using OpenBitSet instead of int[] for doc IDs?
        • If the number of hits is big, we'd gain some RAM back, and be able to cache more entries
        • NOTE: OpenBitSet can only be used for in-order collection only. So we can use that if the wrapped Collector does not support out-of-order
      • Do you think we can modify this Collector to not necessarily wrap another Collector? We have such Collector which stores (in-memory) all matching doc IDs + scores (if required). Those are later fed into several processes that operate on them (e.g. fetch more info from the index etc.). I am thinking, we can make CachingCollector optionally wrap another Collector and then someone can reuse it by setting RAM limit to unlimited (we should have a constant for that) in order to simply collect all matching docs + scores.
      • I think a set of dedicated unit tests for this class alone would be good.

      That's it so far. Perhaps, if we do all of the above, more things will pop up.


        1. LUCENE-3102.patch
          7 kB
          Michael McCandless
        2. LUCENE-3102.patch
          16 kB
          Shai Erera
        3. LUCENE-3102.patch
          16 kB
          Shai Erera
        4. LUCENE-3102-factory.patch
          20 kB
          Shai Erera
        5. LUCENE-3102-nowrap.patch
          5 kB
          Shai Erera
        6. LUCENE-3102-nowrap.patch
          3 kB
          Shai Erera



            shaie Shai Erera
            shaie Shai Erera
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