mikemccand commented on code in PR #16403:
URL: https://github.com/apache/lucene/pull/16403#discussion_r3615127863


##########
lucene/core/src/test/org/apache/lucene/index/TestHnswGraphReuse.java:
##########
@@ -0,0 +1,108 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.index;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+import org.apache.lucene.document.Document;
+import org.apache.lucene.document.KnnFloatVectorField;
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.tests.util.LuceneTestCase;
+import org.apache.lucene.tests.util.TestUtil;
+import org.apache.lucene.util.InfoStream;
+
+/**
+ * Tests that merging deletion-free segments under the default codec reuses 
their existing HNSW

Review Comment:
   Don't we also reuse the HNSW graph if the number of deletions in that 
segment is "small" (by some measure)?  And it also does some incremental work 
to rebuild connections around those deleted nodes or so?  Should we test for 
this case, in case it's (separately) accidentally lost in the future?



##########
lucene/core/src/java/org/apache/lucene/util/hnsw/IncrementalHnswGraphMerger.java:
##########
@@ -108,7 +108,7 @@ public IncrementalHnswGraphMerger addReader(
     if (reader instanceof PerFieldKnnVectorsFormat.FieldsReader 
candidateReader) {
       currKnnVectorsReader = candidateReader.getFieldReader(fieldInfo.name);
     }
-    if (!(reader instanceof HnswGraphProvider)) {
+    if (!(currKnnVectorsReader instanceof HnswGraphProvider)) {

Review Comment:
   Phew the best bugs come down to tiny fixes :)
   
   Did you confirm the new test case fails without these two lines fixes?



##########
lucene/core/src/test/org/apache/lucene/index/TestHnswGraphReuse.java:
##########
@@ -0,0 +1,108 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.index;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+import org.apache.lucene.document.Document;
+import org.apache.lucene.document.KnnFloatVectorField;
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.tests.util.LuceneTestCase;
+import org.apache.lucene.tests.util.TestUtil;
+import org.apache.lucene.util.InfoStream;
+
+/**
+ * Tests that merging deletion-free segments under the default codec reuses 
their existing HNSW
+ * graphs instead of rebuilding the merged graph from scratch. The default 
codec wires vector
+ * readers through {@code PerFieldKnnVectorsFormat.FieldsReader}, the wrapper 
that {@code
+ * IncrementalHnswGraphMerger#addReader} must unwrap for reuse to trigger.
+ *
+ * <p>The assertion keys on the InfoStream message of {@code 
MergingHnswGraphBuilder}, used by the
+ * single threaded merger that the default format selects ({@code 
numMergeWorkers} is 1).
+ */
+public class TestHnswGraphReuse extends LuceneTestCase {

Review Comment:
   Maybe rename to `TestHnswGraphReuseDuringMerge` or so?



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