94828.yaml

pr: 94828
summary: Add new `similarity` field to `knn` clause in `_search`
area: Search
type: feature
issues: []
highlight:
  title: Add new `similarity` field to `knn` clause in `_search`
  body: |-
    This adds a new parameter to `knn` that allows filtering nearest
    neighbor results that are outside a given similarity.

    `num_candidates` and `k` are still required as this controls the
    nearest-neighbor vector search accuracy and exploration. For each shard
    the query will search `num_candidates` and only keep those that are
    within the provided `similarity` boundary, and then finally reduce to
    only the global top `k` as normal.

    For example, when using the `l2_norm` indexed similarity value, this
    could be considered a `radius` post-filter on `knn`.

    relates to: https://github.com/elastic/elasticsearch/issues/84929 &&
    https://github.com/elastic/elasticsearch/pull/93574
  notable: true