Added ef_search to knn search option

Author: ankane

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 5.5.0 (unreleased)
 
-- Added `m` and `ef_construction` options to `knn`
+- Added `m` and `ef_construction` to `knn` index option
+- Added `ef_search` to `knn` search option
 - Fixed exact cosine distance for OpenSearch 2.19+
 - Dropped support for Ruby < 3.2 and Active Record < 7.1
 - Dropped support for Mongoid < 8

lib/searchkick/query.rb

@@ -891,6 +891,7 @@ def set_knn(payload, knn, per_page, offset)
       exact = knn[:exact]
       exact = field_options[:distance].nil? || distance != field_options[:distance] if exact.nil?
       k = per_page + offset
+      ef_search = knn[:ef_search]
       filter = payload.delete(:query)
 
       if distance.nil?
@@ -944,7 +945,7 @@ def set_knn(payload, knn, per_page, offset)
                 vector: vector,
                 k: k,
                 filter: filter
-              }
+              }.merge(ef_search ? {method_parameters: {ef_search: ef_search}} : {})
             }
           }
         end
@@ -992,7 +993,7 @@ def set_knn(payload, knn, per_page, offset)
             query_vector: vector,
             k: k,
             filter: filter
-          }
+          }.merge(ef_search ? {num_candidates: ef_search} : {})
         end
       end
     end

test/knn_test.rb

@@ -198,6 +198,13 @@ def test_explain
     assert_equal "distance must match searchkick options for approximate search", error.message
   end
 
+  def test_ef_search
+    skip if Searchkick.opensearch? && Searchkick.server_below?("2.16.0", true)
+
+    store [{name: "A", embedding: [1, 2, 3]}, {name: "B", embedding: [-1, -2, -3]}, {name: "C"}]
+    assert_order "*", ["A", "B"], knn: {field: :embedding, vector: [1, 2, 3], ef_search: 20}, limit: 10
+  end
+
   private
 
   def assert_approx(approx, field, distance, **knn_options)