Add singleton optimization for DateHistogramAggregator

CHANGELOG.md

@@ -30,6 +30,7 @@ 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 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))
 
 ### Dependencies

server/src/main/java/org/opensearch/search/aggregations/bucket/histogram/DateHistogramAggregator.java

@@ -31,7 +31,9 @@
 
 package org.opensearch.search.aggregations.bucket.histogram;
 
+import org.apache.lucene.index.DocValues;
 import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.index.NumericDocValues;
 import org.apache.lucene.index.SortedNumericDocValues;
 import org.apache.lucene.search.ScoreMode;
 import org.apache.lucene.util.CollectionUtil;
@@ -201,13 +203,28 @@ public LeafBucketCollector getLeafCollector(LeafReaderContext ctx, LeafBucketCol
             return LeafBucketCollector.NO_OP_COLLECTOR;
         }
 
-        SortedNumericDocValues values = valuesSource.longValues(ctx);
+        final SortedNumericDocValues values = valuesSource.longValues(ctx);
+        final NumericDocValues singleton = DocValues.unwrapSingleton(values);
+
+        if (singleton != null) {
+            // Optimized path for single-valued fields
+            return new LeafBucketCollectorBase(sub, values) {
+                @Override
+                public void collect(int doc, long owningBucketOrd) throws IOException {
+                    if (singleton.advanceExact(doc)) {
+                        long value = singleton.longValue();
+                        collectValue(sub, doc, owningBucketOrd, preparedRounding.round(value));
+                    }
+                }
+            };
+        }
+
+        // Original path for multi-valued fields
         return new LeafBucketCollectorBase(sub, values) {
             @Override
             public void collect(int doc, long owningBucketOrd) throws IOException {
                 if (values.advanceExact(doc)) {
                     int valuesCount = values.docValueCount();
-
                     long previousRounded = Long.MIN_VALUE;
                     for (int i = 0; i < valuesCount; ++i) {
                         long value = values.nextValue();
@@ -216,22 +233,26 @@ public void collect(int doc, long owningBucketOrd) throws IOException {
                         if (rounded == previousRounded) {
                             continue;
                         }
-                        if (hardBounds == null || hardBounds.contain(rounded)) {
-                            long bucketOrd = bucketOrds.add(owningBucketOrd, rounded);
-                            if (bucketOrd < 0) { // already seen
-                                bucketOrd = -1 - bucketOrd;
-                                collectExistingBucket(sub, doc, bucketOrd);
-                            } else {
-                                collectBucket(sub, doc, bucketOrd);
-                            }
-                        }
+                        collectValue(sub, doc, owningBucketOrd, rounded);
                         previousRounded = rounded;
                     }
                 }
             }
         };
     }
 
+    private void collectValue(LeafBucketCollector sub, int doc, long owningBucketOrd, long rounded) throws IOException {
+        if (hardBounds == null || hardBounds.contain(rounded)) {
+            long bucketOrd = bucketOrds.add(owningBucketOrd, rounded);
+            if (bucketOrd < 0) { // already seen
+                bucketOrd = -1 - bucketOrd;
+                collectExistingBucket(sub, doc, bucketOrd);
+            } else {
+                collectBucket(sub, doc, bucketOrd);
+            }
+        }
+    }
+
     private String fetchStarTreeCalendarUnit() {
         if (this.rounding.unit() == null) {
             return null;