Are you using SolrCloud or any distributed search ? If you are using just a single Solr instance, LTR should have no problem with pagination. The re-rank involves the top K and then you paginate. So if a document from the original score page 1 ends up in page 3, you will see it at page three. have you verified that : "Say, if an item (Y) from second page is moved to first page after re-ranking, while an item (X) from first page is moved away from the first page. ?" Top K shouldn't start from the "start" parameter, if it does, it is a bug.
The situation change a little with distributed search where you can experiment this behaviour : *Pagination* Let’s explore the scenario on a single Solr node and on a sharded architecture. SINGLE SOLR NODE reRankDocs=15 rows=10 This means each page is composed by 10 results. What happens when we hit the page 2 ? The first 5 documents in the search results will have been rescored and affected by the reranking. The latter 5 documents will preserve the original score and original ranking. e.g. Doc 11 – score= 1.2 Doc 12 – score= 1.1 Doc 13 – score= 1.0 Doc 14 – score= 0.9 Doc 15 – score= 0.8 Doc 16 – score= 5.7 Doc 17 – score= 5.6 Doc 18 – score= 5.5 Doc 19 – score= 4.6 Doc 20 – score= 2.4 This means that score(15) could be < score(16), but document 15 and 16 are still in the expected order. The reason is that the top 15 documents are rescored and reranked and the rest is left unchanged. *SHARDED ARCHITECTURE* reRankDocs=15 rows=10 Shards number=2 When looking for the page 2, Solr will trigger queries to she shards to collect 2 pages per shard : Shard1 : 10 ReRanked docs (page1) + 5 ReRanked docs + 5 OriginalScored docs (page2) Shard2 : 10 ReRanked docs (page1) + 5 ReRanked docs + 5 OriginalScored docs (page2) The the results will be merged, and possibly, original scored search results can top up reranked docs. A possible solution could be to normalise the scores to prevent any possibility that a reranked result is surpassed by original scored ones. Note: The problem is going to happen after you reach rows * page > reRankDocs. In situations when reRankDocs is quite high , the problem will occur only in deep paging. ----- --------------- Alessandro Benedetti Search Consultant, R&D Software Engineer, Director Sease Ltd. - www.sease.io -- Sent from: http://lucene.472066.n3.nabble.com/Solr-User-f472068.html
