Apply default k for knn query eagerly

docs/changelog/118774.yaml

@@ -0,0 +1,5 @@
+pr: 118774
+summary: Apply default k for knn query eagerly
+area: Vector Search
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java

@@ -2006,7 +2006,7 @@ && isNotUnitVector(squaredMagnitude)) {
 
         public Query createKnnQuery(
             VectorData queryVector,
-            Integer k,
+            int k,
             int numCands,
             Float numCandsFactor,
             Query filter,
@@ -2039,7 +2039,7 @@ private boolean needsRescore(Float rescoreOversample) {
 
         private Query createKnnBitQuery(
             byte[] queryVector,
-            Integer k,
+            int k,
             int numCands,
             Query filter,
             Float similarityThreshold,
@@ -2061,7 +2061,7 @@ private Query createKnnBitQuery(
 
         private Query createKnnByteQuery(
             byte[] queryVector,
-            Integer k,
+            int k,
             int numCands,
             Query filter,
             Float similarityThreshold,
@@ -2088,7 +2088,7 @@ private Query createKnnByteQuery(
 
         private Query createKnnFloatQuery(
             float[] queryVector,
-            Integer k,
+            int k,
             int numCands,
             Float numCandsFactor,
             Query filter,

server/src/main/java/org/elasticsearch/search/vectors/KnnVectorQueryBuilder.java

@@ -495,15 +495,13 @@ protected QueryBuilder doIndexMetadataRewrite(QueryRewriteContext context) throw
     @Override
     protected Query doToQuery(SearchExecutionContext context) throws IOException {
         MappedFieldType fieldType = context.getFieldType(fieldName);
-        int requestSize;
-        if (k != null) {
-            requestSize = k;
+        final int k;
+        if (this.k != null) {
+            k = this.k;
         } else {
-            requestSize = context.requestSize() == null || context.requestSize() < 0 ? DEFAULT_SIZE : context.requestSize();
+            k = context.requestSize() == null || context.requestSize() < 0 ? DEFAULT_SIZE : context.requestSize();
         }
-        int adjustedNumCands = numCands == null
-            ? Math.round(Math.min(NUM_CANDS_MULTIPLICATIVE_FACTOR * requestSize, NUM_CANDS_LIMIT))
-            : numCands;
+        int adjustedNumCands = numCands == null ? Math.round(Math.min(NUM_CANDS_MULTIPLICATIVE_FACTOR * k, NUM_CANDS_LIMIT)) : numCands;
         if (fieldType == null) {
             return new MatchNoDocsQuery();
         }

server/src/test/java/org/elasticsearch/search/vectors/AbstractKnnVectorQueryBuilderTestCase.java

@@ -47,7 +47,6 @@
 import java.util.stream.Stream;
 
 import static org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper.NUM_CANDS_OVERSAMPLE_LIMIT;
-import static org.elasticsearch.search.SearchService.DEFAULT_SIZE;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasSize;
@@ -82,7 +81,7 @@ private void checkIndexTypeAndDimensions() {
 
     abstract KnnVectorQueryBuilder createKnnVectorQueryBuilder(
         String fieldName,
-        Integer k,
+        int k,
         int numCands,
         RescoreVectorBuilder rescoreVectorBuilder,
         Float similarity
@@ -138,8 +137,8 @@ protected void initializeAdditionalMappings(MapperService mapperService) throws
     @Override
     protected KnnVectorQueryBuilder doCreateTestQueryBuilder() {
         String fieldName = randomBoolean() ? VECTOR_FIELD : VECTOR_ALIAS_FIELD;
-        Integer k = randomBoolean() ? null : randomIntBetween(1, 100);
-        int numCands = randomIntBetween(k == null ? DEFAULT_SIZE : k + 20, 1000);
+        int k = randomIntBetween(1, 100);
+        int numCands = randomIntBetween(k + 20, 1000);
         KnnVectorQueryBuilder queryBuilder = createKnnVectorQueryBuilder(
             fieldName,
             k,

server/src/test/java/org/elasticsearch/search/vectors/KnnByteVectorQueryBuilderTests.java

@@ -20,7 +20,7 @@ DenseVectorFieldMapper.ElementType elementType() {
     @Override
     protected KnnVectorQueryBuilder createKnnVectorQueryBuilder(
         String fieldName,
-        Integer k,
+        int k,
         int numCands,
         RescoreVectorBuilder rescoreVectorBuilder,
         Float similarity

server/src/test/java/org/elasticsearch/search/vectors/KnnFloatVectorQueryBuilderTests.java

@@ -20,7 +20,7 @@ DenseVectorFieldMapper.ElementType elementType() {
     @Override
     KnnVectorQueryBuilder createKnnVectorQueryBuilder(
         String fieldName,
-        Integer k,
+        int k,
         int numCands,
         RescoreVectorBuilder rescoreVectorBuilder,
         Float similarity