fix and changelog

Signed-off-by: panguixin <[email protected]>

CHANGELOG.md

@@ -24,6 +24,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - New `phone` & `phone-search` analyzer + tokenizer ([#15915](https://github.com/opensearch-project/OpenSearch/pull/15915))
 - Add _list/shards API as paginated alternate to _cat/shards ([#14641](https://github.com/opensearch-project/OpenSearch/pull/14641))
 - Latency and Memory allocation improvements to Multi Term Aggregation queries ([#14993](https://github.com/opensearch-project/OpenSearch/pull/14993))
+- Optimize flat_object type in a BWC way with one phase processing ([#16297](https://github.com/opensearch-project/OpenSearch/pull/16297))
 
 ### Dependencies
 - Bump `com.azure:azure-identity` from 1.13.0 to 1.13.2 ([#15578](https://github.com/opensearch-project/OpenSearch/pull/15578))

server/src/test/java/org/opensearch/index/mapper/FlatObjectFieldTypeTests.java

@@ -9,6 +9,7 @@
 package org.opensearch.index.mapper;
 
 import org.apache.lucene.index.Term;
+import org.apache.lucene.search.DocValuesFieldExistsQuery;
 import org.apache.lucene.search.TermQuery;
 import org.opensearch.Version;
 import org.opensearch.cluster.metadata.IndexMetadata;
@@ -114,7 +115,7 @@ public void testExistsQuery() {
         {
             MappedFieldType ft = getFlatParentFieldType("field");
             // when checking on the flat_object field name "field", check if exist in the field mapper names
-            assertEquals(new TermQuery(new Term(FieldNamesFieldMapper.NAME, "field")), ft.existsQuery(null));
+            assertEquals(new DocValuesFieldExistsQuery("field"), ft.existsQuery(null));
 
             // when checking if a subfield within the flat_object, for example, "field.bar", use term query in the flat_object field
             MappedFieldType dynamicMappedFieldType = new FlatObjectFieldMapper.FlatObjectFieldType("field.bar", ft.name());
@@ -128,7 +129,7 @@ public void testExistsQuery() {
                 false,
                 Collections.emptyMap()
             );
-            assertEquals(new TermQuery(new Term(FieldNamesFieldMapper.NAME, "field")), ft.existsQuery(null));
+            assertEquals(new DocValuesFieldExistsQuery("field"), ft.existsQuery(null));
         }
     }
 }