solr join vs lucene join

Question

I am trying to find how Solr join compares with respect to the Lucene joins. Specifically, if Lucene joins uses any filter cache during the JOIN operation. I looked into code and it seems that in the QParser there is a reference to cache, but I am not sure if it's a filter cache. If somebody has any experience on this, please do share, or please tell me how can I find that.

The Solr join wiki states

Fields or other properties of the documents being joined "from" are not available for use in processing of the resulting set of "to" documents (ie: you can not return fields in the "from" documents as if they were a multivalued field on the "to" documents).

I am finding it hard to understand the above limitation of solr join,does it means that unlike the traditional RDMS joins that can have columns from both the TO and FROM field, solr joins will only have fields from the TO documents ? Is my understanding correct ? If yes, then why this limitation ?

Also, there's some difference with respect to scoring too and towards that the wiki says

The Join query produces constant scores for all documents that match -- scores computed by the nested query for the "from" documents are not available to use in scoring the "to" documents

Does it mean the subquery's score is not available the main query? If so again why solr scoring took this approach ?

If there are any other differences that are worth considering when moving from Lucene join to Solr, please share.

Answer 1

this post is quite old, but I jump on it. Sorry if it's not active any more. To tell the truth, it's far better to avoid the join strategy on solr/lucene. You have to think as object as a whole, joining is much an SQL approch that is not close to the phylosophy of SOLR. Despite that, solr enables very limited joins operations. Take a look to this very good reference join solr lucene ! And also this document about the block join support in solr