Address PR comments and add to CHANGELOG

Signed-off-by: Rishabh Maurya <rishabhmaurya05@gmail.com>

Author: rishabhmaurya

CHANGELOG.md

@@ -21,6 +21,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Tracing for deep search path ([#12103](https://github.com/opensearch-project/OpenSearch/pull/12103))
 - Add explicit dependency to validatePom and generatePom tasks ([#12807](https://github.com/opensearch-project/OpenSearch/pull/12807))
 - Replace configureEach with all for publication iteration ([#12876](https://github.com/opensearch-project/OpenSearch/pull/12876))
+- Derived fields support to derive field values at query time without indexing ([#12569](https://github.com/opensearch-project/OpenSearch/pull/12569))
 
 ### Dependencies
 - Bump `log4j-core` from 2.18.0 to 2.19.0

server/src/main/java/org/opensearch/index/mapper/DerivedFieldMapper.java

@@ -8,8 +8,6 @@
 
 package org.opensearch.index.mapper;
 
-import org.apache.lucene.document.FieldType;
-import org.apache.lucene.index.IndexOptions;
 import org.apache.lucene.index.IndexableField;
 import org.opensearch.core.xcontent.XContentBuilder;
 import org.opensearch.script.Script;
@@ -28,23 +26,6 @@ public class DerivedFieldMapper extends ParametrizedFieldMapper {
 
     public static final String CONTENT_TYPE = "derived";
 
-    /**
-     * Default parameters for the boolean field mapper
-     *
-     * @opensearch.internal
-     */
-    public static class Defaults {
-        public static final FieldType FIELD_TYPE = new FieldType();
-
-        static {
-            FIELD_TYPE.setOmitNorms(true);
-            FIELD_TYPE.setStored(false);
-            FIELD_TYPE.setTokenized(false);
-            FIELD_TYPE.setIndexOptions(IndexOptions.NONE);
-            FIELD_TYPE.freeze();
-        }
-    }
-
     private static DerivedFieldMapper toType(FieldMapper in) {
         return (DerivedFieldMapper) in;
     }

server/src/main/java/org/opensearch/index/mapper/ObjectMapper.java

@@ -294,10 +294,10 @@ protected static boolean parseObjectOrDocumentTypeProperties(
             } else if (fieldName.equals("derived")) {
                 if (fieldNode instanceof Collection && ((Collection) fieldNode).isEmpty()) {
                     // nothing to do here, empty (to support "derived: []" case)
-                } else if (!(fieldNode instanceof Map)) {
-                    throw new OpenSearchParseException("derived must be a map type");
-                } else {
+                } else if (fieldNode instanceof Map) {
                     parseDerived(builder, (Map<String, Object>) fieldNode, parserContext);
+                } else {
+                    throw new OpenSearchParseException("derived must be a map type");
                 }
                 return true;
             } else if (fieldName.equals("properties")) {