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what do you mean by "relevance formula"? You can implement your own Similarity measurement in lucene and change it at query time, so I'm guessing you mean something else?

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You can define arbitrary mathematical functions based on document variables stored in memory, and that you can change in real time. You can have as many of these as you want.

http://indextank.com/documentation/function-definition

(Feedback on this documentation is more than welcome, it's a work in progress!)

please pardon me but I still confused: why can't you do this with the standard lucene? It kind of seems like what solr does with function queries[0]. Is the difference in what the variables are?

http://wiki.apache.org/solr/FunctionQuery

The main difference is that you can update the document variables without fragmenting the index. In Lucene you need to reindex the document (i.e. delete it and write a new one). If you update your relevance variables an average of 10 times per document it can get really fragmented with Lucene.

ah, got it. The point is the document fields are updated in place instead of delete+reinsert. Seems interesting.

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