Thank you for replying. Our documentId field is not the internal _id field of the stored document, but rather an external id of a musical work. You say that search queries are distributed over shards. Yes, but the effect is the same as (replica) shards are distributed over all cluster nodes and so the queries also.
We do not search with the documentId nor the _id field but eg. "iswc:T8000024045" and that query should normally return exactly one match say with documentId="310017001". As this unique (Lucene) document can reside in only one primary shard I assume strongly that the queries for all shards but one will not return any result.
We expect to have less than 20 such secondary identifiers (iswc, isrc, ...). Do you think, that this will have a big impact on internal resources? Does every index need it's own Lucene instance?
On the other hand we expect to have nearly 30 million isrc values and about 200 million dspwc values and we expect some benefit from separate searches.
Kind Regards,
Peter