Bugs with Re-ranking/LtR and ExplainAugmenterFactory

["Sambhav Kothari (BLOOMBERG/ LONDON)" <sk...@bloomberg.net>]

Hello,

Currently, if we use the ExplainAugmenterFactory with LtR, instead of using the model/re-rankers explain method, it uses the default query explain (tf-idf explanation). This happens because the BasicResultContext doesn't wrap the query(https://github.com/apache/lucene-solr/blob/1d85cd783863f75cea133fb9c452302214165a4d/solr/core/src/java/org/apache/solr/response/BasicResultContext.java#L67)  with the RankQuery when its set to context's query, which is then used by the ExplainAugmenterFactory. (https://github.com/apache/lucene-solr/blob/1d85cd783863f75cea133fb9c452302214165a4d/solr/core/src/java/org/apache/solr/response/transform/ExplainAugmenterFactory.java#L111). 

As a result there are discrepancies between queries like - 


http://localhost:8983/solr/collection1/select?q=*:*&collection=collectionName&wt=json&fl=[explain style=nl],score&rq={!ltr model=linear-model}


http://localhost:8983/solr/collection1/select?q=*:*&collection=collectionName&wt=json&fl=score&rq={!ltr model=linear-model}&debugQuery=true

the former outputs the explain from the SimilarityScorer's explain while the latter uses the correct LtR ModelScorer's explain.

There are a few other problems with the explain augmenter - for eg. it doesn't work with grouping (although the other doc transformers like LtR's LTRFeatureLoggerTransformerFactory work with grouping).

Just wanted to discuss these issues before creating tickets on Jira.

Thanks,
Sam