[GitHub] [lucene] mocobeta commented on a change in pull request #238: LUCENE-10016: add support for Knnvector index and query to the demo

[GitBox <gi...@apache.org>]

mocobeta commented on a change in pull request #238:
URL: https://github.com/apache/lucene/pull/238#discussion_r684741332



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File path: lucene/core/src/java/org/apache/lucene/search/KnnVectorQuery.java
##########
@@ -0,0 +1,318 @@
+/*
+ * 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.search;
+
+import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.Comparator;
+import java.util.Objects;
+import org.apache.lucene.codecs.KnnVectorsReader;
+import org.apache.lucene.document.KnnVectorField;
+import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.LeafReaderContext;
+
+/** Uses {@link KnnVectorsReader#search} to perform nearest Neighbour search. */
+public class KnnVectorQuery extends Query {
+
+  private static final TopDocs NO_RESULTS =
+      new TopDocs(new TotalHits(0, TotalHits.Relation.EQUAL_TO), new ScoreDoc[0]);
+
+  private final String field;
+  private final float[] target;
+  private final int k;
+
+  /**
+   * Find the <code>k</code> nearest documents to the target vector according to the vectors in the
+   * given field. <code>target</code> vector.
+   *
+   * @param field a field that has been indexed as a {@link KnnVectorField}.
+   * @param target the target of the search
+   * @param k the number of documents to find
+   * @throws IllegalArgumentException if <code>k</code> is less than 1
+   */
+  public KnnVectorQuery(String field, float[] target, int k) {
+    this.field = field;
+    this.target = target;
+    this.k = k;
+    if (k < 1) {
+      throw new IllegalArgumentException("k must be at least 1, got: " + k);
+    }
+  }
+
+  @Override
+  public Query rewrite(IndexReader reader) throws IOException {
+    int boundedK = Math.min(k, reader.numDocs());
+    TopDocs[] perLeafResults = new TopDocs[reader.leaves().size()];
+    for (LeafReaderContext ctx : reader.leaves()) {
+      // Calculate kPerLeaf as an overestimate of the expected number of the closest k documents in
+      // this leaf
+      int expectedKPerLeaf = Math.max(1, boundedK * ctx.reader().numDocs() / reader.numDocs());
+      // Increase to include 3 std. deviations of a Binomial distribution.
+      int kPerLeaf = (int) (expectedKPerLeaf + 3 * Math.sqrt(expectedKPerLeaf));
+      perLeafResults[ctx.ord] = searchLeaf(ctx, kPerLeaf);
+    }
+    // Merge sort the results
+    TopDocs topK = TopDocs.merge(boundedK, perLeafResults);
+    // re-query any outlier segments (normally there should be none).
+    topK = checkForOutlierSegments(reader, topK, perLeafResults);
+    if (topK.scoreDocs.length == 0) {
+      return new MatchNoDocsQuery();
+    }
+    return createRewrittenQuery(reader, topK);
+  }
+
+  private TopDocs searchLeaf(LeafReaderContext ctx, int kPerLeaf) throws IOException {
+    TopDocs results = ctx.reader().searchNearestVectors(field, target, kPerLeaf);

Review comment:
       I'm wondering if we could (or should) have `searchNearestVectors()` on this Query class, instead of LeafReader? 




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