Skip over metadata fields in the field retrieval API. (#58710)

This avoids unnecessary lookups, since metadata fields don't have _source
values.

Author: jtibshirani

server/src/main/java/org/elasticsearch/search/fetch/subphase/FieldValueRetriever.java

@@ -52,7 +52,7 @@ public static FieldValueRetriever create(MapperService mapperService,
 
             Collection<String> concreteFields = mapperService.simpleMatchToFullName(fieldPattern);
             for (String field : concreteFields) {
-                if (fieldMappers.getMapper(field) != null) {
+                if (fieldMappers.getMapper(field) != null && mapperService.isMetadataField(field) == false) {
                     Set<String> sourcePath = mapperService.sourcePath(field);
                     fields.add(new FieldContext(field, sourcePath, format));
                 }
@@ -62,6 +62,7 @@ public static FieldValueRetriever create(MapperService mapperService,
         return new FieldValueRetriever(fieldMappers, fields);
     }
 
+
     private FieldValueRetriever(DocumentFieldMappers fieldMappers,
                                 List<FieldContext> fieldContexts) {
         this.fieldMappers = fieldMappers;

server/src/test/java/org/elasticsearch/search/fetch/subphase/FieldValueRetrieverTests.java

@@ -93,6 +93,29 @@ public void testNonExistentField() throws IOException {
         assertThat(fields.size(), equalTo(0));
     }
 
+    public void testMetadataFields() throws IOException {
+        MapperService mapperService = createMapperService();
+        XContentBuilder source = XContentFactory.jsonBuilder().startObject()
+            .field("field", "value")
+        .endObject();
+
+        Map<String, DocumentField> fields = retrieveFields(mapperService, source, "_routing");
+        assertTrue(fields.isEmpty());
+    }
+
+    public void testRetrieveAllFields() throws IOException {
+        MapperService mapperService = createMapperService();
+        XContentBuilder source = XContentFactory.jsonBuilder().startObject()
+            .field("field", "value")
+            .startObject("object")
+                .field("field", "other-value")
+            .endObject()
+        .endObject();
+
+        Map<String, DocumentField> fields = retrieveFields(mapperService, source, "*");
+        assertThat(fields.size(), equalTo(2));
+    }
+
     public void testArrayValueMappers() throws IOException {
         MapperService mapperService = createMapperService();