Enable skip_list for @timestamp field or index sort field by default

CHANGELOG.md

@@ -30,6 +30,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add all-active ingestion as docrep equivalent in pull-based ingestion ([#19316](https://github.com/opensearch-project/OpenSearch/pull/19316))
 - Adding logic for histogram aggregation using skiplist ([#19130](https://github.com/opensearch-project/OpenSearch/pull/19130))
 - Add skip_list param for date, scaled float and token count fields ([#19142](https://github.com/opensearch-project/OpenSearch/pull/19142))
+- Enable skip_list for @timestamp field or index sort field by default([#19480](https://github.com/opensearch-project/OpenSearch/pull/19480))
 - Implement GRPC MatchPhrase, MultiMatch queries ([#19449](https://github.com/opensearch-project/OpenSearch/pull/19449))
 - Optimize gRPC transport thread management for improved throughput ([#19278](https://github.com/opensearch-project/OpenSearch/pull/19278))
 - Implement GRPC Boolean query and inject registry for all internal query converters ([#19391](https://github.com/opensearch-project/OpenSearch/pull/19391))

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

@@ -58,6 +58,7 @@
 import org.opensearch.common.util.FeatureFlags;
 import org.opensearch.common.util.LocaleUtils;
 import org.opensearch.common.xcontent.support.XContentMapValues;
+import org.opensearch.index.IndexSortConfig;
 import org.opensearch.index.compositeindex.datacube.DimensionType;
 import org.opensearch.index.fielddata.IndexFieldData;
 import org.opensearch.index.fielddata.IndexNumericFieldData.NumericType;
@@ -753,6 +754,7 @@ public DocValueFormat docValueFormat(@Nullable String format, ZoneId timeZone) {
     private final boolean indexed;
     private final boolean hasDocValues;
     private final boolean skiplist;
+    private final boolean isSkiplistConfigured;
     private final Locale locale;
     private final String format;
     private final String printFormat;
@@ -778,6 +780,7 @@ private DateFieldMapper(
         this.indexed = builder.index.getValue();
         this.hasDocValues = builder.docValues.getValue();
         this.skiplist = builder.skiplist.getValue();
+        this.isSkiplistConfigured = builder.skiplist.isConfigured();
         this.locale = builder.locale.getValue();
         this.format = builder.format.getValue();
         this.printFormat = builder.printFormat.getValue();
@@ -846,7 +849,7 @@ protected void parseCreateField(ParseContext context) throws IOException {
             context.doc().add(new LongPoint(fieldType().name(), timestamp));
         }
         if (hasDocValues) {
-            if (skiplist) {
+            if (skiplist || isSkiplistDefaultEnabled(context.indexSettings().getIndexSortConfig(), fieldType().name())) {
                 context.doc().add(SortedNumericDocValuesField.indexedField(fieldType().name(), timestamp));
             } else {
                 context.doc().add(new SortedNumericDocValuesField(fieldType().name(), timestamp));
@@ -859,6 +862,19 @@ protected void parseCreateField(ParseContext context) throws IOException {
         }
     }
 
+    boolean isSkiplistDefaultEnabled(IndexSortConfig indexSortConfig, String fieldName) {
+        if (!isSkiplistConfigured) {
+            if (indexSortConfig.hasPrimarySortOnField(fieldName)) {
+                return true;
+            }
+            if (DataStreamFieldMapper.Defaults.TIMESTAMP_FIELD.getName().equals(fieldName)) {
+                return true;
+            }
+
+        }
+        return false;
+    }
+
     public Long getNullValue() {
         return nullValue;
     }

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

@@ -44,10 +44,15 @@
 import org.apache.lucene.index.StoredFields;
 import org.apache.lucene.index.VectorEncoding;
 import org.apache.lucene.index.VectorSimilarityFunction;
+import org.opensearch.Version;
+import org.opensearch.cluster.metadata.IndexMetadata;
+import org.opensearch.common.settings.Settings;
 import org.opensearch.common.time.DateFormatter;
 import org.opensearch.common.util.FeatureFlags;
 import org.opensearch.common.xcontent.XContentFactory;
 import org.opensearch.core.xcontent.XContentBuilder;
+import org.opensearch.index.IndexSettings;
+import org.opensearch.index.IndexSortConfig;
 import org.opensearch.index.fieldvisitor.SingleFieldsVisitor;
 import org.opensearch.index.termvectors.TermVectorsService;
 import org.opensearch.search.DocValueFormat;
@@ -833,6 +838,35 @@ public void testSkipListIntegrationMappingDefinitionSerialization() throws IOExc
         assertTrue("skip_list should be true in date_nanos mapper", dateFieldMapperNanos.skiplist());
     }
 
+    public void testIsSkiplistDefaultEnabled() throws IOException {
+        DocumentMapper mapper = createDocumentMapper(fieldMapping(b -> b.field("type", "date")));
+        DateFieldMapper dateFieldMapper = (DateFieldMapper) mapper.mappers().getMapper("field");
+
+        // Test with no index sort and non-timestamp field
+        IndexMetadata noSortindexMetadata = new IndexMetadata.Builder("index").settings(getIndexSettings()).build();
+        IndexSettings noSolrIndexSettings = new IndexSettings(noSortindexMetadata, getIndexSettings());
+        IndexSortConfig noSortConfig = new IndexSortConfig(new IndexSettings(noSortindexMetadata, getIndexSettings()));
+        assertFalse(dateFieldMapper.isSkiplistDefaultEnabled(noSortConfig, "field"));
+
+        // timestamp field
+        assertTrue(dateFieldMapper.isSkiplistDefaultEnabled(noSortConfig, "@timestamp"));
+
+        // Create index settings with an index sort.
+        Settings settings = Settings.builder()
+            .put(IndexMetadata.SETTING_VERSION_CREATED, Version.CURRENT)
+            .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
+            .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 1)
+            .putList("index.sort.field", "field")
+            .build();
+
+        // Test with timestamp field
+        IndexMetadata indexMetadata = new IndexMetadata.Builder("index").settings(settings).build();
+        IndexSettings indexSettings = new IndexSettings(indexMetadata, settings);
+        IndexSortConfig sortConfig = new IndexSortConfig(indexSettings);
+        assertTrue(dateFieldMapper.isSkiplistDefaultEnabled(sortConfig, "field"));
+        assertTrue(dateFieldMapper.isSkiplistDefaultEnabled(sortConfig, "@timestamp"));
+    }
+
     public void testSkipListIntegrationFieldBehaviorConsistency() throws IOException {
         // Test that field behavior is consistent between skip_list enabled and disabled
 

test/framework/src/main/java/org/opensearch/index/mapper/MapperServiceTestCase.java

@@ -80,7 +80,11 @@
 
 public abstract class MapperServiceTestCase extends OpenSearchTestCase {
 
-    protected static final Settings SETTINGS = Settings.builder().put("index.version.created", Version.CURRENT).build();
+    protected static final Settings SETTINGS = Settings.builder()
+        .put("index.version.created", Version.CURRENT)
+        .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
+        .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 1)
+        .build();
 
     protected static final ToXContent.Params INCLUDE_DEFAULTS = new ToXContent.MapParams(
         Collections.singletonMap("include_defaults", "true")