I want to use aggregations over the search result of the knn_search api (because I need facet search on the user interface), but I cannot pass the agg parameter as in the search api. Any suggestions?
Indeed currently (before ES 8.4 version) it is not possible.
But in ES 8.4 we moved knn search inside _search API, so from 8.4 it will be possible to use aggregations on knn search results.