Few issues with CachingCollector
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Key: LUCENE-3102
URL: https://issues.apache.org/jira/browse/LUCENE-3102
Project: Lucene - Java
Issue Type: Bug
Components: contrib/*
Reporter: Shai Erera
Priority: Minor
Fix For: 3.2, 4.0
CachingCollector (introduced in LUCENE-1421) has few issues:
# 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.
# It does not clear cachedScores + cachedSegs upon exceeding RAM limits
# 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
# 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?
Also:
* 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.
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