slow-J commented on code in PR #16271:
URL: https://github.com/apache/lucene/pull/16271#discussion_r3497919704


##########
lucene/core/src/java/org/apache/lucene/search/AbstractKnnVectorQuery.java:
##########
@@ -149,9 +149,64 @@ public Query rewrite(IndexSearcher indexSearcher) throws 
IOException {
       topK = runSearchTasks(tasks, taskExecutor, perLeafResults, 
leafReaderContexts);
     }
     if (topK.scoreDocs.length == 0) {
-      return MatchNoDocsQuery.INSTANCE;
+      return new MatchNoDocsQuery("No documents matched the nearest-neighbor 
search");
     }
-    return DocAndScoreQuery.createDocAndScoreQuery(reader, topK, reentryCount);
+    return DocAndScoreQuery.createDocAndScoreQuery(
+        reader, topK, reentryCount, noMatchExplainer(topK, filterWeight));
+  }
+
+  /** Builds the explainer for documents this query did not collect, capturing 
minTopKScore. */
+  private DocAndScoreQuery.NoMatchExplainer noMatchExplainer(TopDocs topK, 
Weight filterWeight) {
+    // topK is score-descending, so the lowest collected score is the last 
entry.
+    final float minTopKScore = topK.scoreDocs[topK.scoreDocs.length - 1].score;
+    return (context, doc, topN) ->
+        explainNotCollected(context, doc, topN, filterWeight, minTopKScore);
+  }
+
+  /** Explains why a doc was not collected, by recomputing its score. null 
when no vectors. */
+  private Explanation explainNotCollected(
+      LeafReaderContext context, int doc, int topN, Weight filterWeight, float 
minTopKScore)
+      throws IOException {
+    String prefix = "Not in top " + topN + " doc(s): ";
+    FieldInfo fi = context.reader().getFieldInfos().fieldInfo(field);
+    if (fi == null || fi.getVectorDimension() == 0) {
+      return null;
+    }
+    VectorScorer vectorScorer = createVectorScorer(context, fi);
+    if (vectorScorer == null) {
+      return null;
+    }
+    if (vectorScorer.iterator().advance(doc) != doc) {
+      return Explanation.noMatch(prefix + "no vector value in field \"" + 
field + "\"");
+    }
+    if (filterWeight != null && docPassesFilter(filterWeight, context, doc) == 
false) {
+      return Explanation.noMatch(prefix + "excluded by filter");
+    }
+    float score = vectorScorer.score();
+    if (score < minTopKScore) {
+      return Explanation.noMatch(prefix + "score " + score + " < minTopKScore 
" + minTopKScore);

Review Comment:
   Sure :D 
   Edit: done in latest commit



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