Support flattened label field with downsampling. (#118816) (#118936)

If flattened field is configured as non-dimension and non-metric field, then downsampling fails to execute successfully. Downsampling doesn't know how to use the flattened field or how to serialize it. This change addresses this.

Closes #116319

docs/changelog/118816.yaml

@@ -0,0 +1,6 @@
+pr: 118816
+summary: Support flattened field with downsampling
+area: Downsampling
+type: bug
+issues:
+ - 116319

server/src/main/java/org/elasticsearch/index/mapper/flattened/FlattenedFieldMapper.java

@@ -52,6 +52,7 @@ import org.elasticsearch.index.fielddata.plain.SortedSetOrdinalsIndexFieldData;
 import org.elasticsearch.index.mapper.DocumentParserContext;
 import org.elasticsearch.index.mapper.DynamicFieldType;
 import org.elasticsearch.index.mapper.FieldMapper;
+import org.elasticsearch.index.mapper.KeywordFieldMapper;
 import org.elasticsearch.index.mapper.MappedFieldType;
 import org.elasticsearch.index.mapper.Mapper;
 import org.elasticsearch.index.mapper.MapperBuilderContext;
@@ -666,7 +667,7 @@ public final class FlattenedFieldMapper extends FieldMapper {
         private final boolean isDimension;
         private final int ignoreAbove;
 
-        public RootFlattenedFieldType(
+        RootFlattenedFieldType(
             String name,
             boolean indexed,
             boolean hasDocValues,
@@ -678,7 +679,7 @@ public final class FlattenedFieldMapper extends FieldMapper {
             this(name, indexed, hasDocValues, meta, splitQueriesOnWhitespace, eagerGlobalOrdinals, Collections.emptyList(), ignoreAbove);
         }
 
-        public RootFlattenedFieldType(
+        RootFlattenedFieldType(
             String name,
             boolean indexed,
             boolean hasDocValues,
@@ -802,6 +803,10 @@ public final class FlattenedFieldMapper extends FieldMapper {
             return new KeyedFlattenedFieldType(name(), childPath, this);
         }
 
+        public MappedFieldType getKeyedFieldType() {
+            return new KeywordFieldMapper.KeywordFieldType(name() + KEYED_FIELD_SUFFIX);
+        }
+
         @Override
         public boolean isDimension() {
             return isDimension;

server/src/main/java/org/elasticsearch/index/mapper/flattened/FlattenedFieldSyntheticWriterHelper.java

@@ -55,7 +55,7 @@ import java.util.Objects;
  * }`
  *
  */
-class FlattenedFieldSyntheticWriterHelper {
+public class FlattenedFieldSyntheticWriterHelper {
 
     private record Prefix(List<String> prefix) {
 
@@ -225,17 +225,17 @@ class FlattenedFieldSyntheticWriterHelper {
         }
     }
 
-    interface SortedKeyedValues {
+    public interface SortedKeyedValues {
         BytesRef next() throws IOException;
     }
 
     private final SortedKeyedValues sortedKeyedValues;
 
-    FlattenedFieldSyntheticWriterHelper(final SortedKeyedValues sortedKeyedValues) {
+    public FlattenedFieldSyntheticWriterHelper(final SortedKeyedValues sortedKeyedValues) {
         this.sortedKeyedValues = sortedKeyedValues;
     }
 
-    void write(final XContentBuilder b) throws IOException {
+    public void write(final XContentBuilder b) throws IOException {
         KeyValue curr = new KeyValue(sortedKeyedValues.next());
         KeyValue prev = KeyValue.EMPTY;
         final List<String> values = new ArrayList<>();

x-pack/plugin/downsample/qa/rest/src/yamlRestTest/resources/rest-api-spec/test/downsample/70_flattened_field_type.yml

@@ -0,0 +1,307 @@
+---
+"A flattened label field":
+  - do:
+      indices.create:
+        index: source_index
+        body:
+          settings:
+            number_of_shards: 1
+            index:
+              mode: time_series
+              routing_path: [ metricset, k8s.pod.uid ]
+              time_series:
+                start_time: 2021-04-28T00:00:00Z
+                end_time: 2021-04-29T00:00:00Z
+          mappings:
+            subobjects: false
+            properties:
+              "@timestamp":
+                type: date
+              metricset:
+                type: keyword
+                time_series_dimension: true
+              k8s:
+                properties:
+                  pod:
+                    properties:
+                      uid:
+                        type: keyword
+                        time_series_dimension: true
+                      name:
+                        type: keyword
+                  agent:
+                    type: flattened
+                  value:
+                    type: long
+                    time_series_metric: gauge
+
+  - do:
+      bulk:
+        refresh: true
+        index: source_index
+        body:
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:04.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.4" }, "value": 10 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:24.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.5" }, "value": 20 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T20:50:44.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.6" }, "value": 12 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T20:51:04.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.7" }, "value": 15 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:03.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.7" }, "value": 9 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:23.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.8" }, "value": 16 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T19:50:53.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.9" }, "value": 25 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T19:51:03.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.10" }, "value": 17 }}'
+
+  - do:
+      indices.put_settings:
+        index: source_index
+        body:
+          index.blocks.write: true
+
+  - do:
+      indices.downsample:
+        index: source_index
+        target_index: target_index
+        body: >
+          {
+            "fixed_interval": "1h"
+          }
+  - is_true: acknowledged
+
+  - do:
+      search:
+        index: target_index
+        body:
+          sort: [ "_tsid", "@timestamp" ]
+
+  - length: { hits.hits: 4 }
+  - match: { hits.hits.0._source._doc_count: 2 }
+  - match: { hits.hits.0._source.k8s\.pod\.uid: df3145b3-0563-4d3b-a0f7-897eb2876ea9 }
+  - match: { hits.hits.0._source.@timestamp: 2021-04-28T18:00:00.000Z }
+  - match: { hits.hits.0._source.k8s\.agent: { "id": "second", "version": "2.1.8" } }
+
+  - match: { hits.hits.1._source._doc_count: 2 }
+  - match: { hits.hits.1._source.k8s\.pod\.uid: df3145b3-0563-4d3b-a0f7-897eb2876ea9 }
+  - match: { hits.hits.1._source.@timestamp: 2021-04-28T19:00:00.000Z }
+  - match: { hits.hits.1._source.k8s\.agent: { "id": "second", "version": "2.1.10" } }
+
+  - match: { hits.hits.2._source._doc_count: 2 }
+  - match: { hits.hits.2._source.k8s\.pod\.uid: 947e4ced-1786-4e53-9e0c-5c447e959507 }
+  - match: { hits.hits.2._source.@timestamp: 2021-04-28T18:00:00.000Z }
+  - match: { hits.hits.2._source.k8s\.agent: { "id": "first", "version": "2.0.5" } }
+
+  - match: { hits.hits.3._source._doc_count: 2 }
+  - match: { hits.hits.3._source.k8s\.pod\.uid: 947e4ced-1786-4e53-9e0c-5c447e959507 }
+  - match: { hits.hits.3._source.@timestamp: 2021-04-28T20:00:00.000Z }
+  - match: { hits.hits.3._source.k8s\.agent: { "id": "first", "version": "2.0.7" } }
+
+---
+"A flattened label field with no doc values":
+  - do:
+      indices.create:
+        index: source_index
+        body:
+          settings:
+            number_of_shards: 1
+            index:
+              mode: time_series
+              routing_path: [ metricset, k8s.pod.uid ]
+              time_series:
+                start_time: 2021-04-28T00:00:00Z
+                end_time: 2021-04-29T00:00:00Z
+          mappings:
+            subobjects: false
+            properties:
+              "@timestamp":
+                type: date
+              metricset:
+                type: keyword
+                time_series_dimension: true
+              k8s:
+                properties:
+                  pod:
+                    properties:
+                      uid:
+                        type: keyword
+                        time_series_dimension: true
+                      name:
+                        type: keyword
+                  agent:
+                    type: flattened
+                    doc_values: false
+                  value:
+                    type: long
+                    time_series_metric: gauge
+
+  - do:
+      bulk:
+        refresh: true
+        index: source_index
+        body:
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:04.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.4" }, "value": 10 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:24.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.5" }, "value": 20 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T20:50:44.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.6" }, "value": 12 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T20:51:04.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.7" }, "value": 15 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:03.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.7" }, "value": 9 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:23.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.8" }, "value": 16 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T19:50:53.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.9" }, "value": 25 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T19:51:03.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.10" }, "value": 17 }}'
+
+  - do:
+      indices.put_settings:
+        index: source_index
+        body:
+          index.blocks.write: true
+
+  - do:
+      indices.downsample:
+        index: source_index
+        target_index: target_index
+        body: >
+          {
+            "fixed_interval": "1h"
+          }
+  - is_true: acknowledged
+
+  - do:
+      search:
+        index: target_index
+        body:
+          sort: [ "_tsid", "@timestamp" ]
+
+  - length: { hits.hits: 4 }
+  - match: { hits.hits.0._source._doc_count: 2 }
+  - match: { hits.hits.0._source.k8s\.pod\.uid: df3145b3-0563-4d3b-a0f7-897eb2876ea9 }
+  - match: { hits.hits.0._source.@timestamp: 2021-04-28T18:00:00.000Z }
+  - is_false: hits.hits.0._source.k8s\.agent
+
+  - match: { hits.hits.1._source._doc_count: 2 }
+  - match: { hits.hits.1._source.k8s\.pod\.uid: df3145b3-0563-4d3b-a0f7-897eb2876ea9 }
+  - match: { hits.hits.1._source.@timestamp: 2021-04-28T19:00:00.000Z }
+  - is_false: hits.hits.1._source.k8s\.agent
+
+  - match: { hits.hits.2._source._doc_count: 2 }
+  - match: { hits.hits.2._source.k8s\.pod\.uid: 947e4ced-1786-4e53-9e0c-5c447e959507 }
+  - match: { hits.hits.2._source.@timestamp: 2021-04-28T18:00:00.000Z }
+  - is_false: hits.hits.2._source.k8s\.agent
+
+  - match: { hits.hits.3._source._doc_count: 2 }
+  - match: { hits.hits.3._source.k8s\.pod\.uid: 947e4ced-1786-4e53-9e0c-5c447e959507 }
+  - match: { hits.hits.3._source.@timestamp: 2021-04-28T20:00:00.000Z }
+  - is_false: hits.hits.3._source.k8s\.agent
+
+---
+"A flattened label field with mixed content":
+  - do:
+      indices.create:
+        index: source_index
+        body:
+          settings:
+            number_of_shards: 1
+            index:
+              mode: time_series
+              routing_path: [ metricset, k8s.pod.uid ]
+              time_series:
+                start_time: 2021-04-28T00:00:00Z
+                end_time: 2021-04-29T00:00:00Z
+          mappings:
+            subobjects: false
+            properties:
+              "@timestamp":
+                type: date
+              metricset:
+                type: keyword
+                time_series_dimension: true
+              k8s:
+                properties:
+                  pod:
+                    properties:
+                      uid:
+                        type: keyword
+                        time_series_dimension: true
+                      name:
+                        type: keyword
+                  agent:
+                    type: flattened
+                    null_value: my_null_value
+                  value:
+                    type: long
+                    time_series_metric: gauge
+
+  - do:
+      bulk:
+        refresh: true
+        index: source_index
+        body:
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:04.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.4", "versions": ["1", "2", "3"], "optional_version": null, "dotted.version": "1.1", "numeric_version": 11 }, "value": 10 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:24.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.5", "versions": ["1", "2", "3"], "optional_version": null, "dotted.version": "1.1", "numeric_version": 11}, "value": 20 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T20:50:44.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.6", "versions": ["1", "2", "3"], "optional_version": null, "dotted.version": "1.1", "numeric_version": 11}, "value": 12 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T20:51:04.467Z", "metricset": "pod", "k8s": {"pod": {"name": "cat", "uid":"947e4ced-1786-4e53-9e0c-5c447e959507"}, "agent": { "id": "first", "version": "2.0.7", "versions": ["1", "2", "3"], "optional_version": null, "dotted.version": "1.1", "numeric_version": 11}, "value": 15 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:03.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.7", "versions": ["1", "2", "3"], "optional_version": null, "dotted.version": "1.1", "numeric_version": 11}, "value": 9 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T18:50:23.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.8", "versions": ["1", "2", "3"], "optional_version": null, "dotted.version": "1.1", "numeric_version": 11}, "value": 16 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T19:50:53.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.9", "versions": ["1", "2", "3"], "optional_version": null, "dotted.version": "1.1", "numeric_version": 11}, "value": 25 }}'
+          - '{"index": {}}'
+          - '{"@timestamp": "2021-04-28T19:51:03.142Z", "metricset": "pod", "k8s": {"pod": {"name": "dog", "uid":"df3145b3-0563-4d3b-a0f7-897eb2876ea9"}, "agent": { "id": "second", "version": "2.1.10", "versions": ["1", "2", "3"], "optional_version": null, "dotted.version": "1.1", "numeric_version": 11}, "value": 17 }}'
+
+  - do:
+      indices.put_settings:
+        index: source_index
+        body:
+          index.blocks.write: true
+
+  - do:
+      indices.downsample:
+        index: source_index
+        target_index: target_index
+        body: >
+          {
+            "fixed_interval": "1h"
+          }
+  - is_true: acknowledged
+
+  - do:
+      search:
+        index: target_index
+        body:
+          sort: [ "_tsid", "@timestamp" ]
+
+  - length: { hits.hits: 4 }
+  - match: { hits.hits.0._source._doc_count: 2 }
+  - match: { hits.hits.0._source.k8s\.pod\.uid: df3145b3-0563-4d3b-a0f7-897eb2876ea9 }
+  - match: { hits.hits.0._source.@timestamp: 2021-04-28T18:00:00.000Z }
+  - match: { hits.hits.0._source.k8s\.agent: { "id": "second", "version": "2.1.8", "versions": ["1", "2", "3"], "dotted": {"version": "1.1"}, "numeric_version": "11", optional_version: "my_null_value" } }
+
+  - match: { hits.hits.1._source._doc_count: 2 }
+  - match: { hits.hits.1._source.k8s\.pod\.uid: df3145b3-0563-4d3b-a0f7-897eb2876ea9 }
+  - match: { hits.hits.1._source.@timestamp: 2021-04-28T19:00:00.000Z }
+  - match: { hits.hits.1._source.k8s\.agent: { "id": "second", "version": "2.1.10", "versions": ["1", "2", "3"], "dotted": {"version": "1.1"}, "numeric_version": "11", optional_version: "my_null_value" } }
+
+  - match: { hits.hits.2._source._doc_count: 2 }
+  - match: { hits.hits.2._source.k8s\.pod\.uid: 947e4ced-1786-4e53-9e0c-5c447e959507 }
+  - match: { hits.hits.2._source.@timestamp: 2021-04-28T18:00:00.000Z }
+  - match: { hits.hits.2._source.k8s\.agent: { "id": "first", "version": "2.0.5", "versions": ["1", "2", "3"], "dotted": {"version": "1.1"}, "numeric_version": "11", optional_version: "my_null_value" } }
+
+  - match: { hits.hits.3._source._doc_count: 2 }
+  - match: { hits.hits.3._source.k8s\.pod\.uid: 947e4ced-1786-4e53-9e0c-5c447e959507 }
+  - match: { hits.hits.3._source.@timestamp: 2021-04-28T20:00:00.000Z }
+  - match: { hits.hits.3._source.k8s\.agent: { "id": "first", "version": "2.0.7", "versions": ["1", "2", "3"], "dotted": {"version": "1.1"}, "numeric_version": "11", optional_version: "my_null_value" } }

x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/FieldValueFetcher.java

@@ -12,6 +12,7 @@ import org.elasticsearch.index.fielddata.FormattedDocValues;
 import org.elasticsearch.index.fielddata.IndexFieldData;
 import org.elasticsearch.index.mapper.MappedFieldType;
 import org.elasticsearch.index.mapper.NumberFieldMapper;
+import org.elasticsearch.index.mapper.flattened.FlattenedFieldMapper;
 import org.elasticsearch.index.query.SearchExecutionContext;
 import org.elasticsearch.search.DocValueFormat;
 import org.elasticsearch.xpack.aggregatemetric.mapper.AggregateDoubleMetricFieldMapper;
@@ -65,6 +66,8 @@ class FieldValueFetcher {
             // If field is not a metric, we downsample it as a label
             if ("histogram".equals(fieldType.typeName())) {
                 return new LabelFieldProducer.HistogramLastLabelFieldProducer(name());
+            } else if ("flattened".equals(fieldType.typeName())) {
+                return new LabelFieldProducer.FlattenedLastValueFieldProducer(name());
             }
             return new LabelFieldProducer.LabelLastValueFieldProducer(name());
         }
@@ -90,7 +93,13 @@ class FieldValueFetcher {
                 }
             } else {
                 if (context.fieldExistsInIndex(field)) {
-                    final IndexFieldData<?> fieldData = context.getForField(fieldType, MappedFieldType.FielddataOperation.SEARCH);
+                    final IndexFieldData<?> fieldData;
+                    if (fieldType instanceof FlattenedFieldMapper.RootFlattenedFieldType flattenedFieldType) {
+                        var keyedFieldType = flattenedFieldType.getKeyedFieldType();
+                        fieldData = context.getForField(keyedFieldType, MappedFieldType.FielddataOperation.SEARCH);
+                    } else {
+                        fieldData = context.getForField(fieldType, MappedFieldType.FielddataOperation.SEARCH);
+                    }
                     final String fieldName = context.isMultiField(field)
                         ? fieldType.name().substring(0, fieldType.name().lastIndexOf('.'))
                         : fieldType.name();

x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/LabelFieldProducer.java

@@ -7,8 +7,10 @@
 
 package org.elasticsearch.xpack.downsample;
 
+import org.apache.lucene.util.BytesRef;
 import org.elasticsearch.index.fielddata.FormattedDocValues;
 import org.elasticsearch.index.fielddata.HistogramValue;
+import org.elasticsearch.index.mapper.flattened.FlattenedFieldSyntheticWriterHelper;
 import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xpack.aggregatemetric.mapper.AggregateDoubleMetricFieldMapper.Metric;
 
@@ -141,14 +143,14 @@ abstract class LabelFieldProducer extends AbstractDownsampleFieldProducer {
         }
     }
 
-    static class AggregateMetricFieldProducer extends LabelLastValueFieldProducer {
+    static final class AggregateMetricFieldProducer extends LabelLastValueFieldProducer {
 
         AggregateMetricFieldProducer(String name, Metric metric) {
             super(name, new LastValueLabel(metric.name()));
         }
     }
 
-    public static class HistogramLastLabelFieldProducer extends LabelLastValueFieldProducer {
+    static final class HistogramLastLabelFieldProducer extends LabelLastValueFieldProducer {
         HistogramLastLabelFieldProducer(String name) {
             super(name);
         }
@@ -167,4 +169,40 @@ abstract class LabelFieldProducer extends AbstractDownsampleFieldProducer {
             }
         }
     }
+
+    static final class FlattenedLastValueFieldProducer extends LabelLastValueFieldProducer {
+
+        FlattenedLastValueFieldProducer(String name) {
+            super(name);
+        }
+
+        @Override
+        public void write(XContentBuilder builder) throws IOException {
+            if (isEmpty() == false) {
+                builder.startObject(name());
+
+                var value = label.get();
+                List<BytesRef> list;
+                if (value instanceof Object[] values) {
+                    list = new ArrayList<>(values.length);
+                    for (Object v : values) {
+                        list.add(new BytesRef(v.toString()));
+                    }
+                } else {
+                    list = List.of(new BytesRef(value.toString()));
+                }
+
+                var iterator = list.iterator();
+                var helper = new FlattenedFieldSyntheticWriterHelper(() -> {
+                    if (iterator.hasNext()) {
+                        return iterator.next();
+                    } else {
+                        return null;
+                    }
+                });
+                helper.write(builder);
+                builder.endObject();
+            }
+        }
+    }
 }

x-pack/plugin/downsample/src/test/java/org/elasticsearch/xpack/downsample/LabelFieldProducerTests.java

@@ -7,10 +7,18 @@
 
 package org.elasticsearch.xpack.downsample;
 
+import org.elasticsearch.common.Strings;
 import org.elasticsearch.index.fielddata.FormattedDocValues;
 import org.elasticsearch.search.aggregations.AggregatorTestCase;
+import org.elasticsearch.xcontent.XContentBuilder;
+import org.elasticsearch.xcontent.XContentType;
 
+import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.util.Iterator;
+import java.util.List;
+
+import static org.hamcrest.Matchers.equalTo;
 
 public class LabelFieldProducerTests extends AggregatorTestCase {
 
@@ -93,4 +101,50 @@ public class LabelFieldProducerTests extends AggregatorTestCase {
         assertTrue(producer.isEmpty());
         assertNull(producer.label().get());
     }
+
+    public void testFlattenedLastValueFieldProducer() throws IOException {
+        var producer = new LabelFieldProducer.FlattenedLastValueFieldProducer("dummy");
+        assertTrue(producer.isEmpty());
+        assertEquals("dummy", producer.name());
+        assertEquals("last_value", producer.label().name());
+
+        var bytes = List.of("a\0value_a", "b\0value_b", "c\0value_c", "d\0value_d");
+        var docValues = new FormattedDocValues() {
+
+            Iterator<String> iterator = bytes.iterator();
+
+            @Override
+            public boolean advanceExact(int docId) {
+                return true;
+            }
+
+            @Override
+            public int docValueCount() {
+                return bytes.size();
+            }
+
+            @Override
+            public Object nextValue() {
+                return iterator.next();
+            }
+        };
+
+        producer.collect(docValues, 1);
+        assertFalse(producer.isEmpty());
+        assertEquals("a\0value_a", (((Object[]) producer.label().get())[0]).toString());
+        assertEquals("b\0value_b", (((Object[]) producer.label().get())[1]).toString());
+        assertEquals("c\0value_c", (((Object[]) producer.label().get())[2]).toString());
+        assertEquals("d\0value_d", (((Object[]) producer.label().get())[3]).toString());
+
+        var builder = new XContentBuilder(XContentType.JSON.xContent(), new ByteArrayOutputStream());
+        builder.startObject();
+        producer.write(builder);
+        builder.endObject();
+        var content = Strings.toString(builder);
+        assertThat(content, equalTo("{\"dummy\":{\"a\":\"value_a\",\"b\":\"value_b\",\"c\":\"value_c\",\"d\":\"value_d\"}}"));
+
+        producer.reset();
+        assertTrue(producer.isEmpty());
+        assertNull(producer.label().get());
+    }
 }