Support DISTINCT select on single field projection

I think this is now possible in recent versions of Elasticsearch, and it’s called “collapsing”:

On a related note, if you can use Hibernate Search 6 aggregations to retrieve a list of the top N distinct terms of a given field for all matching documents. See here for an example.

That being said, you wouldn't get pagination.

It's not hard to do in Lucene - assuming the limitations I mentioned in the description: it has to be a projection on the terms only, and even then on a single field.

We can't possibly use this information as a filtering clause for other components, such as to select entities.

I don't know about Elasticsearch though - happy to postpone this.

After looking into it a bit more, it doesn't seem to be possible in Elasticsearch... So I guess we will have a hard time doing it in Lucene too. See https://stackoverflow.com/questions/27776582/aggregation-sorting-pagination-in-elastic-search

Maybe the best solution would be to avoid the problem altogether, advising users querying index A that need a DISTINCT on some related entity B's ID to index that entity B instead, and to rely on nested fields for advanced predicates on entity A.

Adding as a pre-requisite. To support DISTINCT properly together with limits and offsets (pagination), we need aggregation features in the backend.