[jira] [Commented] (SOLR-12343) JSON Field Facet refinement can return incorrect counts/stats for sorted buckets

["Hoss Man (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SOLR-12343?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16516506#comment-16516506 ] 

Hoss Man commented on SOLR-12343:
---------------------------------


Updated patch with more tests and some code tweaks based on a few things the new tests caught.

Still outstanding is the question of the new BitSets I added...

{quote}
* buckets now keep track of how many shards contributed to them ...
** there's a nocommit in here about the possibility of re-using the {{Context.sawShard}} BitSet instead – but i couldn't wrap my head around an efficient way to do it so i punted
* ...buckets are excluded if a bucket doesn't have contributions from as many shards as the FacetField...
** again, i needed a new BitSet in at the FacetField level to count the shards – because Context.numShards may include shards that never return any results for the facet (ie: empty shard) so they never merge any data at all)
{quote}

I _think_ it should be possible to re-implement the {{FacetBucket.getNumShardsMerged()}} method (i added) using {{Context.sawShard}} by using {{sawShard.get(bucketNum * numShards, bucketNum * numShards + numShards)}} to take a "slice" of the BitSet just for the current bucket and then look at it's cardinality.  the added cost of taking the slice only for buckets being considered in sorted order is probably a better trade off them the overhead of creating a new BitSet for every FacetBucket even if they are never considered for the response.

But I still don't see anyway to efficiently figure out the "shards that participated" info needed at the {{FacetField}} level using the existing {{sawShard}} BitSet -- particularly with the changes I had to make to account for the case where a shard has docs participating in a facet, but not matching any buckets (see {{testSortedSubFacetRefinementWhenParentOnlyReturnedByOneShard}} ).  Fortunately that's just one new BitSet per FacetField instance (not per bucket).

----

I'll look at refactoring {{FacetBucket.getNumShardsMerged()}} to use {{Context.sawShard}} soon.


> JSON Field Facet refinement can return incorrect counts/stats for sorted buckets
> --------------------------------------------------------------------------------
>
>                 Key: SOLR-12343
>                 URL: https://issues.apache.org/jira/browse/SOLR-12343
>             Project: Solr
>          Issue Type: Bug
>      Security Level: Public(Default Security Level. Issues are Public) 
>            Reporter: Hoss Man
>            Priority: Major
>         Attachments: SOLR-12343.patch, SOLR-12343.patch, SOLR-12343.patch
>
>
> The way JSON Facet's simple refinement "re-sorts" buckets after refinement can cause _refined_ buckets to be "bumped out" of the topN based on the refined counts/stats depending on the sort - causing _unrefined_ buckets originally discounted in phase#2 to bubble up into the topN and be returned to clients *with inaccurate counts/stats*
> The simplest way to demonstrate this bug (in some data sets) is with a {{sort: 'count asc'}} facet:
>  * assume shard1 returns termX & termY in phase#1 because they have very low shard1 counts
>  ** but *not* returned at all by shard2, because these terms both have very high shard2 counts.
>  * Assume termX has a slightly lower shard1 count then termY, such that:
>  ** termX "makes the cut" off for the limit=N topN buckets
>  ** termY does not make the cut, and is the "N+1" known bucket at the end of phase#1
>  * termX then gets included in the phase#2 refinement request against shard2
>  ** termX now has a much higher _known_ total count then termY
>  ** the coordinator now sorts termX "worse" in the sorted list of buckets then termY
>  ** which causes termY to bubble up into the topN
>  * termY is ultimately included in the final result _with incomplete count/stat/sub-facet data_ instead of termX
>  ** this is all indepenent of the possibility that termY may actually have a significantly higher total count then termX across the entire collection
>  ** the key problem is that all/most of the other terms returned to the client have counts/stats that are the cumulation of all shards, but termY only has the contributions from shard1
> Important Notes:
>  * This scenerio can happen regardless of the amount of overrequest used. Additional overrequest just increases the number of "extra" terms needed in the index with "better" sort values then termX & termY in shard2
>  * {{sort: 'count asc'}} is not just an exceptional/pathelogical case:
>  ** any function sort where additional data provided shards during refinement can cause a bucket to "sort worse" can also cause this problem.
>  ** Examples: {{sum(price_i) asc}} , {{min(price_i) desc}} , {{avg(price_i) asc|desc}} , etc...



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