optimize fetching when an empty array of fields is provided, in such a case, we only need the uid (id and type)

modules/elasticsearch/src/main/java/org/elasticsearch/index/mapper/UidFieldSelector.java

@@ -0,0 +1,38 @@
+/*
+ * Licensed to Elastic Search and Shay Banon under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. Elastic Search licenses this
+ * file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.index.mapper;
+
+import org.apache.lucene.document.FieldSelector;
+import org.apache.lucene.document.FieldSelectorResult;
+
+/**
+ * An optimized field selector that loads just the uid.
+ *
+ * @author kimchy (shay.banon)
+ */
+public class UidFieldSelector implements FieldSelector {
+
+    @Override public FieldSelectorResult accept(String fieldName) {
+        if (UidFieldMapper.NAME.equals(fieldName)) {
+            return FieldSelectorResult.LOAD_AND_BREAK;
+        }
+        return FieldSelectorResult.NO_LOAD;
+    }
+}

modules/elasticsearch/src/main/java/org/elasticsearch/search/fetch/FetchPhase.java

@@ -156,6 +156,10 @@ public class FetchPhase implements SearchPhase {
             return new UidAndSourceFieldSelector();
         }
 
+        if (context.fieldNames().length == 0) {
+            return new UidFieldSelector();
+        }
+
         FieldMappersFieldSelector fieldSelector = new FieldMappersFieldSelector();
         for (String fieldName : context.fieldNames()) {
             FieldMappers x = context.mapperService().smartNameFieldMappers(fieldName);