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From "Per Steffensen (JIRA)" <j...@apache.org>
Subject [jira] [Created] (SOLR-3765) Wrong handling of documents with same id in cross collection searches
Date Wed, 29 Aug 2012 06:36:07 GMT
Per Steffensen created SOLR-3765:
------------------------------------

             Summary: Wrong handling of documents with same id in cross collection searches
                 Key: SOLR-3765
                 URL: https://issues.apache.org/jira/browse/SOLR-3765
             Project: Solr
          Issue Type: Bug
          Components: search, SolrCloud
    Affects Versions: 4.0
         Environment: Self-build version of Solr fra 4.x branch (revision )
            Reporter: Per Steffensen


Dialog with myself from solr-users mailing list:

Per Steffensen skrev:
> Hi
>
> Due to what we have seen in recent tests I got in doubt how Solr search is actually supposed
to behave
>
> * Searching with "distrib=true&q=*:*&rows=10&collection=x,y,z&sort=timestamp
asc"
> ** Is Solr supposed to return the 10 documents with the lowest timestamp across all documents
in all slices of collection x, y and z, or is it supposed to just pick 10 random documents
from those slices and just sort those 10 randomly selected documents?
> ** Put in another way - is this search supposed to be consistent, returning exactly the
same set of documents when performed several times (no documents are updated between consecutive
searches)?
Fortunately I believe the answer is, that it ought to "return the 10 documents with the lowest
timestamp across all documents in all slices of collection x, y and Z". The reason I asked
was because I got different responses for consecutive simular requests. Now I believe it can
be explained by the bug described below. I guess they you do cross-collection/shard searches,
the "request-handling" Solr forwards the query to all involved shards simultanious and merges
sub-results into the final result as they are returned from the shards. Because of the "consider
documents with same id as the same document even though the come from different collections"-bug
it is kinda random (depending on which shards responds first/last), for a given id, what collection
the document with that specific id is taken from. And if documents with the same id from different
collections has different timestamp it is random where that document ends up in the final
sorted result.

So i believe this inconsistency can be explained by the bug described below.
>
> * A search returns a "numFound"-field telling how many documents all in all matches the
search-criteria, even though not all those documents are returned by the search. It is a crazy
question to ask, but I will do it anyway because we actually see a problem with this. Isnt
it correct that two searches which only differs on the "rows"-number (documents to be returned)
should always return the same value for "numFound"?
Well I found out myself what the problem is (or seems to be) - see:
http://lucene.472066.n3.nabble.com/Changing-value-of-start-parameter-affects-numFound-td2460645.html
http://lucene.472066.n3.nabble.com/numFound-inconsistent-for-different-rows-param-td3997269.html
http://lucene.472066.n3.nabble.com/Solr-v3-5-0-numFound-changes-when-paging-through-results-on-8-shard-cluster-td3990400.html

Until 4.0 this "bug" could be "ignored" because it was ok for a cross-shards search to consider
documents with identical id's as dublets and therefore only returning/counting one of them.
It is still, in 4.0, ok within the same collection, but across collections identical id's
should not be considered dublicates and should not reduce documents returned/counted. So i
believe this "feature" has now become a bug in 4.0 when it comes to cross-collections searches.
>
> Thanks!
>
> Regards, Steff
>



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