OK, it sounds like this is indeed expected (thanks Adrien, Greg, Mike,
Patrick!), that the estimated hit count (GREATER_THAN_OR_EQUAL_TO) can
indeed change if the segment order is different but otherwise identical
documents are in each segment (yes, with different global doc ids, so the
true identical document case would tie break by those).
That was/is surprising to me -- I thought segment order did not alter
anything at search time (except the tie-break-by-global-docid). I thought
each segment was searched, independently, by itself, and then results
"merged" together. But this has become more complex/interesting with time
:)
So is this because BMW will use the best results from previous segments to
optimize/improve skipping while searching future segments? I.e. some sort
of state carries over from having searched past segments, to better
optimize searching the next segment? This makes total sense (that we would
optimize this way). I guess this also means that concurrent search ("thread
per slice/segment") would not be able to optimize with BMW as well?
Mike McCandless
http://blog.mikemccandless.com
On Sun, May 23, 2021 at 8:24 AM Michael Sokolov <msoko...@gmail.com> wrote:
> Agree with Adrien and Greg, approximate hit counts may vary. That global
> docid tiebreaking may affect the actual results is an interesting
> observation too, thanks Adrien. It seems to me the best way forward is to
> relax the check. Depending on how the index is constructed the difference
> could be quite large. Eg if the index is sorted and all the best documents
> are in one segment, we could end up with quite different counts; they could
> get many multiples different if you collect single threaded.
>
> On Fri, May 21, 2021, 2:22 PM Greg Miller <gsmil...@gmail.com> wrote:
>
>> Hi Patrick-
>>
>> Interesting finds and deep dive!
>>
>> Just to confirm, in the cases of differing hit counts that you've
>> observed, these are TotalHits with a Relation of
>> "GREATER_THAN_OR_EQUAL_TO" right? Never cases where the Relation is
>> "EQUAL_TO"? I ask because that would change my opinion of
>> whether-or-not this is concerning. If we're proving a hit count with
>> "equal to" semantics, it should be correct and shouldn't change based
>> on segment ordering, etc. But, on the other hand, if we're just
>> providing a floor (i.e., "there are at least this many hits"), then
>> I'm not particularly concerned by this (assuming that floor is
>> actually correct in call cases and there aren't fewer hits).
>>
>> Cheers,
>> -Greg
>>
>> On Fri, May 21, 2021 at 2:25 AM Adrien Grand <jpou...@gmail.com> wrote:
>> >
>> > Hi Patrick,
>> >
>> >
>> > Why do you feel weird about the fact that segment order impacts the hit
>> count estimation? It feels ok to me, especially as segment order has deeper
>> implications, e.g. you could get different top hits given that Lucene uses
>> the global doc ID as a tie breaker for documents that produce the same
>> score.
>> >
>> > Could IndexRearranger write segments in deterministic order in the
>> segments file to improve reproducibility? Or could your application
>> configure leaf order explicitly? (LUCENE-9507)
>> >
>> > Le ven. 21 mai 2021 à 08:29, Patrick Zhai <zhai7...@gmail.com> a écrit
>> :
>> >>
>> >> Hi folks
>> >>
>> >> For the past few weeks I've been working with Mike McCandless to use
>> the recent introduced IndexRearranger to replace the old way of guarantee
>> deterministic index -- using a single index thread and a LogDocMergePolicy.
>> >>
>> >> In the progress we found out that with two concurrently built but
>> rearranged indexes, the estimation hit count will show a small difference.
>> I've carefully checked the index and found they're almost the same but the
>> segment order is different (index 1 might be segment 1,2,3,4,5 while index
>> 2 might be segment 2,1,3,5,4 where nth segment contains exactly the same
>> documents and sorted using the same criteria). So I suspected the segment
>> order impacted the hit count estimation and to confirm that I turned off
>> the concurrency of rearranger so that it will always create segments in
>> order. The result proved my theory that the segment order was impacting the
>> hit count estimation.
>> >>
>> >> Later on I did some investigation and found in TopScoreDocCollector we
>> do have logic of updating the global minScore so I guess that's where makes
>> the difference. Mike and I both feel a little weird that segment order will
>> affect the hit count estimation, so just want to
>> >> 1. See whether there's any chance we could improve the API or
>> documentation
>> >> 2. Seek some advice on how should we tackle the problem, obviously we
>> don't want rearranger to execute on only 1 thread (since we use it for
>> speed!), currently what we're considering is to relax the check for hit
>> count estimation, but maybe there's a better way?
>> >>
>> >> Best
>> >> Patrick
>>
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