pass filterbitset as null and add integ tests.

Signed-off-by: Wei Wang <[email protected]>

src/main/java/org/opensearch/knn/index/query/FilterIdsSelector.java

@@ -78,7 +78,10 @@ public enum FilterIdsSelectorType {
     public static FilterIdsSelector getFilterIdSelector(final BitSet filterIdsBitSet, final int cardinality) throws IOException {
         long[] filterIds;
         FilterIdsSelector.FilterIdsSelectorType filterType;
-        if (filterIdsBitSet instanceof FixedBitSet) {
+        if (filterIdsBitSet == null) {
+            filterIds = null;
+            filterType = FilterIdsSelector.FilterIdsSelectorType.BITMAP;
+        } else if (filterIdsBitSet instanceof FixedBitSet) {
             /**
              * When filterIds is dense filter, using fixed bitset
              */

src/main/java/org/opensearch/knn/index/query/KNNWeight.java

@@ -146,13 +146,14 @@ public PerLeafResult searchLeaf(LeafReaderContext context, int k) throws IOExcep
             Map<Integer, Float> result = doExactSearch(context, new BitSetIterator(filterBitSet, cardinality), cardinality, k);
             return new PerLeafResult(filterWeight == null ? null : filterBitSet, result);
         }
+
         /*
-         * If filters match all docs in this segment, then there is no need to do any extra step
-         * and should directly do ANN Search*/
-        if (filterWeight != null && cardinality == maxDoc) {
-            return new PerLeafResult(new FixedBitSet(0), doANNSearch(context, new FixedBitSet(0), 0, k));
-        }
-        Map<Integer, Float> docIdsToScoreMap = doANNSearch(context, filterBitSet, cardinality, k);
+         * If filters match all docs in this segment, then null should be passed as filterBitSet
+         * so that it will not do a bitset look up in bottom search layer.
+         */
+        final BitSet annFilter = filterWeight != null && cardinality == maxDoc ? null : filterBitSet;
+        final Map<Integer, Float> docIdsToScoreMap = doANNSearch(context, annFilter, cardinality, k);
+
         // See whether we have to perform exact search based on approx search results
         // This is required if there are no native engine files or if approximate search returned
         // results less than K, though we have more than k filtered docs
@@ -161,7 +162,7 @@ public PerLeafResult searchLeaf(LeafReaderContext context, int k) throws IOExcep
             Map<Integer, Float> result = doExactSearch(context, docs, cardinality, k);
             return new PerLeafResult(filterWeight == null ? null : filterBitSet, result);
         }
-        return new PerLeafResult(filterWeight == null ? null : filterBitSet, docIdsToScoreMap);
+        return new PerLeafResult((filterWeight == null || cardinality == maxDoc) ? null : filterBitSet, docIdsToScoreMap);
     }
 
     private BitSet getFilteredDocsBitSet(final LeafReaderContext ctx) throws IOException {
@@ -327,7 +328,6 @@ private Map<Integer, Float> doANNSearch(
         // Now that we have the allocation, we need to readLock it
         indexAllocation.readLock();
         indexAllocation.incRef();
-
         try {
             if (indexAllocation.isClosed()) {
                 throw new RuntimeException("Index has already been closed");

src/test/java/org/opensearch/knn/index/query/KNNWeightTests.java

@@ -769,16 +769,7 @@ public void testANNWithFilterQuery_whenFiltersMatchAllDocs_thenSuccess() {
         }
 
         jniServiceMockedStatic.when(
-            () -> JNIService.queryIndex(
-                anyLong(),
-                eq(QUERY_VECTOR),
-                eq(k),
-                eq(HNSW_METHOD_PARAMETERS),
-                any(),
-                eq(new FixedBitSet(0).getBits()),
-                anyInt(),
-                any()
-            )
+            () -> JNIService.queryIndex(anyLong(), eq(QUERY_VECTOR), eq(k), eq(HNSW_METHOD_PARAMETERS), any(), eq(null), anyInt(), any())
         ).thenReturn(getFilteredKNNQueryResults());
 
         final Bits liveDocsBits = mock(Bits.class);

src/test/java/org/opensearch/knn/integ/FilteredSearchANNSearchIT.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.knn.integ;
+
+import com.google.common.collect.ImmutableMap;
+import lombok.SneakyThrows;
+import lombok.extern.log4j.Log4j2;
+import org.apache.hc.core5.http.io.entity.EntityUtils;
+import org.opensearch.client.Response;
+import org.opensearch.common.settings.Settings;
+import org.opensearch.knn.KNNJsonQueryBuilder;
+import org.opensearch.knn.KNNRestTestCase;
+import org.opensearch.knn.index.KNNSettings;
+import java.util.List;
+
+import static org.opensearch.knn.common.KNNConstants.FAISS_NAME;
+import static org.opensearch.knn.common.KNNConstants.METHOD_HNSW;
+
+@Log4j2
+public class FilteredSearchANNSearchIT extends KNNRestTestCase {
+    @SneakyThrows
+    public void testFilteredSearchWithFaissHnsw_whenFiltersMatchAllDocs_thenReturnCorrectResults() {
+        String filterFieldName = "color";
+        final int expectResultSize = randomIntBetween(1, 3);
+        final String filterValue = "red";
+        createKnnIndex(INDEX_NAME, getKNNDefaultIndexSettings(), createKnnIndexMapping(FIELD_NAME, 3, METHOD_HNSW, FAISS_NAME));
+
+        // ingest 4 vector docs into the index with the same field {"color": "red"}
+        for (int i = 0; i < 4; i++) {
+            addKnnDocWithAttributes(String.valueOf(i), new float[] { i, i, i }, ImmutableMap.of(filterFieldName, filterValue));
+        }
+
+        refreshIndex(INDEX_NAME);
+        forceMergeKnnIndex(INDEX_NAME);
+
+        updateIndexSettings(INDEX_NAME, Settings.builder().put(KNNSettings.ADVANCED_FILTERED_EXACT_SEARCH_THRESHOLD, 0));
+
+        Float[] queryVector = { 3f, 3f, 3f };
+        // All docs in one segment will match the filters value
+        String query = KNNJsonQueryBuilder.builder()
+            .fieldName(FIELD_NAME)
+            .vector(queryVector)
+            .k(expectResultSize)
+            .filterFieldName(filterFieldName)
+            .filterValue(filterValue)
+            .build()
+            .getQueryString();
+        Response response = searchKNNIndex(INDEX_NAME, query, expectResultSize);
+        String entity = EntityUtils.toString(response.getEntity());
+        List<String> docIds = parseIds(entity);
+        assertEquals(expectResultSize, docIds.size());
+        assertEquals(expectResultSize, parseTotalSearchHits(entity));
+    }
+}