Fix minmax normalizer handling of single-doc result sets (elastic#128689) (elastic#128756)

(cherry picked from commit adda402)

# Conflicts:
#	x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/RankRRFFeatures.java

Author: Mikep86

docs/changelog/128689.yaml

@@ -0,0 +1,5 @@
+pr: 128689
+summary: Fix minmax normalizer handling of single-doc result sets
+area: Search
+type: bug
+issues: []

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/RankRRFFeatures.java

@@ -14,6 +14,7 @@
 import java.util.Set;
 
 import static org.elasticsearch.search.retriever.CompoundRetrieverBuilder.INNER_RETRIEVERS_FILTER_SUPPORT;
+import static org.elasticsearch.xpack.rank.linear.MinMaxScoreNormalizer.LINEAR_RETRIEVER_MINMAX_SINGLE_DOC_FIX;
 import static org.elasticsearch.xpack.rank.rrf.RRFRetrieverBuilder.RRF_RETRIEVER_COMPOSITION_SUPPORTED;
 
 /**
@@ -30,6 +31,6 @@ public Set<NodeFeature> getFeatures() {
 
     @Override
     public Set<NodeFeature> getTestFeatures() {
-        return Set.of(INNER_RETRIEVERS_FILTER_SUPPORT);
+        return Set.of(INNER_RETRIEVERS_FILTER_SUPPORT, LINEAR_RETRIEVER_MINMAX_SINGLE_DOC_FIX);
     }
 }

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/linear/MinMaxScoreNormalizer.java

@@ -8,11 +8,13 @@
 package org.elasticsearch.xpack.rank.linear;
 
 import org.apache.lucene.search.ScoreDoc;
+import org.elasticsearch.features.NodeFeature;
 
 public class MinMaxScoreNormalizer extends ScoreNormalizer {
-
     public static final MinMaxScoreNormalizer INSTANCE = new MinMaxScoreNormalizer();
 
+    public static final NodeFeature LINEAR_RETRIEVER_MINMAX_SINGLE_DOC_FIX = new NodeFeature("linear_retriever.minmax_single_doc_fix");
+
     public static final String NAME = "minmax";
 
     private static final float EPSILON = 1e-6f;
@@ -54,7 +56,9 @@ public ScoreDoc[] normalizeScores(ScoreDoc[] docs) {
         for (int i = 0; i < docs.length; i++) {
             float score;
             if (minEqualsMax) {
-                score = min;
+                // This can happen if there is only one doc in the result set or if all docs have nearly equivalent scores
+                // (i.e. within epsilon). In this case, assign every doc the max normalized score.
+                score = 1.0f;
             } else {
                 score = (docs[i].score - min) / (max - min);
             }

x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/linear/10_linear_retriever.yml

@@ -930,7 +930,7 @@ setup:
 
 
 ---
-"linear retriever with custom sort and score for nested retrievers":
+"linear retriever with custom sort for nested retrievers":
   - do:
       search:
         index: test
@@ -949,17 +949,18 @@ setup:
                                 {
                                   term: {
                                     keyword: {
-                                      value: "one"  # this will give doc 1 a normalized score of 10 because min == max
+                                      value: "one"
                                     }
                                   }
                                 },
                                 {
                                   term: {
                                     keyword: {
-                                      value: "two" # this will give doc 2 a normalized score of 10 because min == max
+                                      value: "two"
                                     }
                                   }
-                                } ]
+                                }
+                              ]
                             }
                           },
                           boost: 10.0
@@ -1058,11 +1059,11 @@ setup:
           size: 2
 
   - match: { hits.total.value: 3 }
-  - length: {hits.hits: 2}
+  - length: { hits.hits: 2 }
   - match: { hits.hits.0._id: "2" }
-  - close_to: { hits.hits.0._score: { value: 10.5, error: 0.001 } }
+  - close_to: { hits.hits.0._score: { value: 1.5, error: 0.001 } }
   - match: { hits.hits.1._id: "1" }
-  - match: { hits.hits.1._score: 10 }
+  - match: { hits.hits.1._score: 1 }
 
 ---
 "should throw when rank_window_size is negative":
@@ -1102,3 +1103,36 @@ setup:
               ]
               rank_window_size: -10
   - match: { status: 400 }
+
+---
+"minmax normalization properly handles a single doc result set":
+  - requires:
+      cluster_features: [ "linear_retriever.minmax_single_doc_fix" ]
+      reason: "Fix bug where minmax normalizer would emit unnormalized score when handling a single doc result set"
+
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            linear:
+              retrievers: [
+                {
+                  retriever: {
+                    standard: {
+                      query: {
+                        term: {
+                          "keyword": {
+                            "value": "one"
+                          }
+                        }
+                      }
+                    }
+                  },
+                  normalizer: "minmax"
+                }
+              ]
+
+  - match: { hits.total.value: 1 }
+  - match: { hits.hits.0._id: "1" }
+  - match: { hits.hits.0._score: 1.0 }