This is a good summary.

I've build an interesting NN-based model, and I've been thinking about using some of the early layers as feature for a search engine.

The obvious thing to do is to dump the feature vectors into Elastic or Solr or something.

Ideally what I want is to:

1) Put 1024 dimensional vectors of floats into the index

2) Use plain cosine distance as the distance metric

2b) Customize the distance metric (or preferably use a preexisting and optimized earth-mover-distance implementation).

My initial Googling indicated that 1 and 2 are harder than I expected - it seems both Elastic and Solr don't have good representations for vectors, and assume you want BM25 or TF-IDF for your ranking.

Surely I'm missing something? Not super-keen on having to drop back to using Lucene.