Return float[] instead of List<Double> in valueFetcher (elastic#126702) (elastic#126811)

We are currently having to hold in heap big list of Double objects which can take big amounts of heap. With this change 
we can reduce the heap usage by 7x.

Author: iverase

docs/changelog/126702.yaml

@@ -0,0 +1,5 @@
+pr: 126702
+summary: "Return float[] instead of List<Double> in `valueFetcher`"
+area: Search
+type: enhancement
+issues: []

x-pack/plugin/rank-vectors/src/main/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldMapper.java

@@ -180,7 +180,20 @@ public ValueFetcher valueFetcher(SearchExecutionContext context, String format)
             return new ArraySourceValueFetcher(name(), context) {
                 @Override
                 protected Object parseSourceValue(Object value) {
-                    return value;
+                    List<?> outerList = (List<?>) value;
+                    List<Object> vectors = new ArrayList<>(outerList.size());
+                    for (Object o : outerList) {
+                        if (o instanceof List<?> innerList) {
+                            float[] vector = new float[innerList.size()];
+                            for (int i = 0; i < vector.length; i++) {
+                                vector[i] = ((Number) innerList.get(i)).floatValue();
+                            }
+                            vectors.add(vector);
+                        } else {
+                            vectors.add(o);
+                        }
+                    }
+                    return vectors;
                 }
             };
         }

x-pack/plugin/rank-vectors/src/test/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldMapperTests.java

@@ -383,19 +383,10 @@ protected void assertFetch(MapperService mapperService, String field, Object val
             switch (denseVectorFieldType.getElementType()) {
                 case BYTE -> assumeFalse("byte element type testing not currently added", false);
                 case FLOAT -> {
-                    List<float[]> fetchedFloatsList = new ArrayList<>();
-                    for (var f : fromNative) {
-                        float[] fetchedFloats = new float[denseVectorFieldType.getVectorDimensions()];
-                        assert f instanceof List;
-                        List<?> vector = (List<?>) f;
-                        int i = 0;
-                        for (Object v : vector) {
-                            assert v instanceof Number;
-                            fetchedFloats[i++] = ((Number) v).floatValue();
-                        }
-                        fetchedFloatsList.add(fetchedFloats);
+                    float[][] fetchedFloats = new float[fromNative.size()][];
+                    for (int i = 0; i < fromNative.size(); i++) {
+                        fetchedFloats[i] = (float[]) fromNative.get(i);
                     }
-                    float[][] fetchedFloats = fetchedFloatsList.toArray(new float[0][]);
                     assertThat("fetching " + value, fetchedFloats, equalTo(value));
                 }
             }

x-pack/plugin/rank-vectors/src/test/java/org/elasticsearch/xpack/rank/vectors/mapper/RankVectorsFieldTypeTests.java

@@ -19,10 +19,13 @@
 
 import java.io.IOException;
 import java.util.Collections;
+import java.util.HexFormat;
 import java.util.List;
 import java.util.Set;
 
 import static org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper.BBQ_MIN_DIMS;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.iterableWithSize;
 
 public class RankVectorsFieldTypeTests extends FieldTypeTestCase {
 
@@ -86,9 +89,16 @@ public void testDocValueFormat() {
 
     public void testFetchSourceValue() throws IOException {
         RankVectorsFieldType fft = createFloatFieldType();
-        List<List<Double>> vector = List.of(List.of(0.0, 1.0, 2.0, 3.0, 4.0, 6.0));
-        assertEquals(vector, fetchSourceValue(fft, vector));
+        List<List<Double>> vectorFromXContent = List.of(List.of(0.0, 1.0, 2.0, 3.0, 4.0, 6.0));
+        List<float[]> vector = List.of(new float[] { 0.0f, 1.0f, 2.0f, 3.0f, 4.0f, 6.0f });
+        assertThat(fetchSourceValue(fft, vectorFromXContent), iterableWithSize(1));
+        assertThat(fetchSourceValue(fft, vectorFromXContent).get(0), equalTo(vector.get(0)));
         RankVectorsFieldType bft = createByteFieldType();
-        assertEquals(vector, fetchSourceValue(bft, vector));
+        assertThat(fetchSourceValue(bft, vectorFromXContent), iterableWithSize(1));
+        assertThat(fetchSourceValue(bft, vectorFromXContent).get(0), equalTo(vector.get(0)));
+        String hexStr = HexFormat.of().formatHex(new byte[] { 0, 1, 2, 3, 4, 6 });
+        List<String> hexVecs = List.of(hexStr);
+        assertThat(fetchSourceValue(bft, hexVecs), iterableWithSize(1));
+        assertThat(fetchSourceValue(bft, hexVecs).get(0), equalTo(hexStr));
     }
 }