Fix flaky testFilteredQueryStrategy by allow drop to brute-force (elastic#129590)

Author: benwtrent

server/src/internalClusterTest/java/org/elasticsearch/search/query/VectorIT.java

@@ -9,6 +9,7 @@
 
 package org.elasticsearch.search.query;
 
+import org.apache.lucene.util.VectorUtil;
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper;
@@ -30,10 +31,11 @@ public class VectorIT extends ESIntegTestCase {
     private static final String VECTOR_FIELD = "vector";
     private static final String NUM_ID_FIELD = "num_id";
 
-    private static void randomVector(float[] vector) {
+    private static void randomVector(float[] vector, int constant) {
         for (int i = 0; i < vector.length; i++) {
-            vector[i] = randomFloat();
+            vector[i] = randomFloat() * constant;
         }
+        VectorUtil.l2normalize(vector);
     }
 
     @Before
@@ -43,6 +45,7 @@ public void setup() throws IOException {
             .startObject("properties")
             .startObject(VECTOR_FIELD)
             .field("type", "dense_vector")
+            .field("similarity", "dot_product")
             .startObject("index_options")
             .field("type", "hnsw")
             .endObject()
@@ -59,20 +62,21 @@ public void setup() throws IOException {
             .build();
         prepareCreate(INDEX_NAME).setMapping(mapping).setSettings(settings).get();
         ensureGreen(INDEX_NAME);
+        float[] vector = new float[16];
         for (int i = 0; i < 150; i++) {
-            float[] vector = new float[8];
-            randomVector(vector);
+            randomVector(vector, i % 25 + 1);
             prepareIndex(INDEX_NAME).setId(Integer.toString(i)).setSource(VECTOR_FIELD, vector, NUM_ID_FIELD, i).get();
         }
         forceMerge(true);
         refresh(INDEX_NAME);
     }
 
     public void testFilteredQueryStrategy() {
-        float[] vector = new float[8];
-        randomVector(vector);
+        float[] vector = new float[16];
+        randomVector(vector, 25);
+        int upperLimit = 35;
         var query = new KnnSearchBuilder(VECTOR_FIELD, vector, 1, 1, null, null).addFilterQuery(
-            QueryBuilders.rangeQuery(NUM_ID_FIELD).lte(30)
+            QueryBuilders.rangeQuery(NUM_ID_FIELD).lte(35)
         );
         assertResponse(client().prepareSearch(INDEX_NAME).setKnnSearch(List.of(query)).setSize(1).setProfile(true), acornResponse -> {
             assertNotEquals(0, acornResponse.getHits().getHits().length);
@@ -116,6 +120,8 @@ public void testFilteredQueryStrategy() {
                 assertTrue(
                     "fanoutVectorOps [" + fanoutVectorOpsSum + "] is not gt acornVectorOps [" + vectorOpsSum + "]",
                     fanoutVectorOpsSum > vectorOpsSum
+                        // if both switch to brute-force due to excessive exploration, they will both equal to upperLimit
+                        || (fanoutVectorOpsSum == vectorOpsSum && vectorOpsSum == upperLimit + 1)
                 );
             });
         });