Add "emitEmptyBuckets" parameter to the "Bucket" function.

benchmarks/src/main/java/org/elasticsearch/benchmark/compute/operator/AggregatorBenchmark.java

@@ -191,11 +191,12 @@ private static Operator operator(DriverContext driverContext, String grouping, S
                 new BlockHash.GroupSpec(2, ElementType.BYTES_REF)
             );
             case TOP_N_LONGS -> List.of(
-                new BlockHash.GroupSpec(0, ElementType.LONG, null, new BlockHash.TopNDef(0, true, true, TOP_N_LIMIT))
+                new BlockHash.GroupSpec(0, ElementType.LONG, null, new BlockHash.TopNDef(0, true, true, TOP_N_LIMIT), null)
             );
             default -> throw new IllegalArgumentException("unsupported grouping [" + grouping + "]");
         };
         return new HashAggregationOperator(
+            groups,
             List.of(supplier(op, dataType, filter).groupingAggregatorFactory(AggregatorMode.SINGLE, List.of(groups.size()))),
             () -> BlockHash.build(groups, driverContext.blockFactory(), 16 * 1024, false),
             driverContext

benchmarks/src/main/java/org/elasticsearch/benchmark/compute/operator/ValuesAggregatorBenchmark.java

@@ -122,6 +122,7 @@ private static Operator operator(DriverContext driverContext, int groups, String
         }
         List<BlockHash.GroupSpec> groupSpec = List.of(new BlockHash.GroupSpec(0, ElementType.LONG));
         return new HashAggregationOperator(
+            groupSpec,
             List.of(supplier(dataType).groupingAggregatorFactory(mode, List.of(1))),
             () -> BlockHash.build(groupSpec, driverContext.blockFactory(), 16 * 1024, false),
             driverContext

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/GroupingAggregator.java

@@ -23,6 +23,10 @@ public class GroupingAggregator implements Releasable {
 
     private final AggregatorMode mode;
 
+    public AggregatorMode getMode() {
+        return mode;
+    }
+
     public interface Factory extends Function<DriverContext, GroupingAggregator>, Describable {}
 
     public GroupingAggregator(GroupingAggregatorFunction aggregatorFunction, AggregatorMode mode) {

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/aggregation/blockhash/BlockHash.java

@@ -127,6 +127,12 @@ public abstract class BlockHash implements Releasable, SeenGroupIds {
      */
     public record TopNDef(int order, boolean asc, boolean nullsFirst, int limit) {}
 
+    public interface EmptyBucketGenerator {
+        int getEmptyBucketCount();
+
+        void generate(Block.Builder blockBuilder);
+    }
+
     /**
      * Configuration for a BlockHash group spec that is doing text categorization.
      */
@@ -137,13 +143,19 @@ public enum OutputFormat {
         }
     }
 
-    public record GroupSpec(int channel, ElementType elementType, @Nullable CategorizeDef categorizeDef, @Nullable TopNDef topNDef) {
+    public record GroupSpec(
+        int channel,
+        ElementType elementType,
+        @Nullable CategorizeDef categorizeDef,
+        @Nullable TopNDef topNDef,
+        @Nullable EmptyBucketGenerator emptyBucketGenerator
+    ) {
         public GroupSpec(int channel, ElementType elementType) {
-            this(channel, elementType, null, null);
+            this(channel, elementType, null, null, null);
         }
 
         public GroupSpec(int channel, ElementType elementType, CategorizeDef categorizeDef) {
-            this(channel, elementType, categorizeDef, null);
+            this(channel, elementType, categorizeDef, null, null);
         }
 
         public boolean isCategorize() {