[Aggregations] Optimize singleton handling in GlobalOrdinalValuesSource

CHANGELOG.md

@@ -31,6 +31,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Changed
 - Migrate BC libs to their FIPS counterparts ([#14912](https://github.com/opensearch-project/OpenSearch/pull/14912))
 - Increase the floor segment size to 16MB ([#17699](https://github.com/opensearch-project/OpenSearch/pull/17699))
+- Unwrap singleton DocValues in global ordinal value source of composite histogram aggregation ([#17740](https://github.com/opensearch-project/OpenSearch/pull/17740))
 - Unwrap singleton DocValues in date histogram aggregation. ([#17643](https://github.com/opensearch-project/OpenSearch/pull/17643))
 - Introduce 512 byte limit to search and ingest pipeline IDs ([#17786](https://github.com/opensearch-project/OpenSearch/pull/17786))
 

server/src/main/java/org/opensearch/search/aggregations/bucket/composite/GlobalOrdinalValuesSource.java

@@ -32,8 +32,10 @@
 
 package org.opensearch.search.aggregations.bucket.composite;
 
+import org.apache.lucene.index.DocValues;
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.index.SortedDocValues;
 import org.apache.lucene.index.SortedSetDocValues;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.Query;
@@ -171,6 +173,26 @@ LeafBucketCollector getLeafCollector(LeafReaderContext context, LeafBucketCollec
         if (lookup == null) {
             initLookup(dvs);
         }
+
+        // unwrapSingleton() returns non-null only if the field is single-valued
+        final SortedDocValues singleton = DocValues.unwrapSingleton(dvs);
+
+        // Direct ordinal access for single-valued fields
+        if (singleton != null) {
+            return new LeafBucketCollector() {
+                @Override
+                public void collect(int doc, long bucket) throws IOException {
+                    if (singleton.advanceExact(doc)) {
+                        currentValue = singleton.ordValue();
+                        next.collect(doc, bucket);
+                    } else if (missingBucket) {
+                        currentValue = -1;
+                        next.collect(doc, bucket);
+                    }
+                }
+            };
+        }
+
         return new LeafBucketCollector() {
             @Override
             public void collect(int doc, long bucket) throws IOException {