From 993951430b9a797f90de0c8aae47b97820e669ea Mon Sep 17 00:00:00 2001
From: Dominik Schmidt <dev@dominik-schmidt.de>
Date: Mon, 5 Oct 2026 12:10:55 +0200
Subject: [PATCH] feat(search): add nested aggregations

Sub-aggregations on both engines: bleve folds child buckets below their parent through the same collector, OpenSearch nests them natively, so a request stays one search per space. The proto gains sub_aggregations on the option and on the bucket; merge, finalize, validation and the bucket limit walk every level, results are positional at every level, a metric has no buckets to nest in. AGG-10 to AGG-15, 20, 35, 40 and 44 in the parity suite, AGG-14 as a known OpenSearch divergence.
---
 .../opencloud/services/search/v0/search.pb.go | 348 ++++++++++--------
 .../services/search/v0/search.swagger.json    |  14 +
 .../opencloud/services/search/v0/search.proto |   6 +
 services/graph/pkg/service/v0/searchquery.go  |  19 +-
 .../pkg/service/v0/searchquery_mapping.go     |  13 +-
 .../graph/pkg/service/v0/searchquery_test.go  |  10 +-
 .../search/pkg/aggregation/aggregation.go     |   7 +-
 .../pkg/aggregation/aggregation_test.go       |  16 +
 services/search/pkg/aggregation/merge.go      |  19 +-
 services/search/pkg/aggregation/merge_test.go |  96 ++++-
 services/search/pkg/aggregation/validate.go   |   6 +
 .../search/pkg/aggregation/validate_test.go   |  10 +
 services/search/pkg/bleve/aggregations.go     |  44 ++-
 .../pkg/opensearch/internal/aggs/aggs.go      |  25 +-
 .../pkg/opensearch/internal/aggs/aggs_test.go |  59 +++
 services/search/pkg/parity/README.md          |  10 +
 .../search/pkg/parity/aggregations_test.go    | 108 +++++-
 17 files changed, 603 insertions(+), 207 deletions(-)

diff --git a/protogen/gen/opencloud/services/search/v0/search.pb.go b/protogen/gen/opencloud/services/search/v0/search.pb.go
index d968b2e364..7f5a84efdb 100644
--- a/protogen/gen/opencloud/services/search/v0/search.pb.go
+++ b/protogen/gen/opencloud/services/search/v0/search.pb.go
@@ -478,6 +478,9 @@ type AggregationOption struct {
 	// rather than a bucket aggregation; the corresponding AggregationResult
 	// carries `metric` instead of `buckets`.
 	MetricDefinition *MetricDefinition `protobuf:"bytes,4,opt,name=metric_definition,json=metricDefinition,proto3" json:"metric_definition,omitempty"`
+	// Optional. Nested aggregations computed within each bucket of this
+	// aggregation.
+	SubAggregations []*AggregationOption `protobuf:"bytes,5,rep,name=sub_aggregations,json=subAggregations,proto3" json:"sub_aggregations,omitempty"`
 }
 
 func (x *AggregationOption) Reset() {
@@ -540,6 +543,13 @@ func (x *AggregationOption) GetMetricDefinition() *MetricDefinition {
 	return nil
 }
 
+func (x *AggregationOption) GetSubAggregations() []*AggregationOption {
+	if x != nil {
+		return x.SubAggregations
+	}
+	return nil
+}
+
 type MetricDefinition struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -886,6 +896,9 @@ type Bucket struct {
 
 	Key   string `protobuf:"bytes,1,opt,name=key,proto3" json:"key,omitempty"`
 	Count int64  `protobuf:"varint,2,opt,name=count,proto3" json:"count,omitempty"`
+	// Nested aggregation results, one per sub_aggregation of the parent
+	// AggregationOption, at the same position.
+	SubAggregations []*AggregationResult `protobuf:"bytes,3,rep,name=sub_aggregations,json=subAggregations,proto3" json:"sub_aggregations,omitempty"`
 }
 
 func (x *Bucket) Reset() {
@@ -934,6 +947,13 @@ func (x *Bucket) GetCount() int64 {
 	return 0
 }
 
+func (x *Bucket) GetSubAggregations() []*AggregationResult {
+	if x != nil {
+		return x.SubAggregations
+	}
+	return nil
+}
+
 type IndexSpaceRequest struct {
 	state         protoimpl.MessageState
 	sizeCache     protoimpl.SizeCache
@@ -1179,7 +1199,7 @@ var file_opencloud_services_search_v0_search_proto_rawDesc = []byte{
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 var (
@@ -1385,24 +1417,26 @@ var file_opencloud_services_search_v0_search_proto_depIdxs = []int32{
 	10, // 7: opencloud.services.search.v0.SearchIndexResponse.aggregations:type_name -> opencloud.services.search.v0.AggregationResult
 	8,  // 8: opencloud.services.search.v0.AggregationOption.bucket_definition:type_name -> opencloud.services.search.v0.BucketDefinition
 	7,  // 9: opencloud.services.search.v0.AggregationOption.metric_definition:type_name -> opencloud.services.search.v0.MetricDefinition
-	0,  // 10: opencloud.services.search.v0.MetricDefinition.kind:type_name -> opencloud.services.search.v0.MetricKind
-	1,  // 11: opencloud.services.search.v0.BucketDefinition.sort_by:type_name -> opencloud.services.search.v0.BucketSortBy
-	9,  // 12: opencloud.services.search.v0.BucketDefinition.ranges:type_name -> opencloud.services.search.v0.BucketRange
-	12, // 13: opencloud.services.search.v0.AggregationResult.buckets:type_name -> opencloud.services.search.v0.Bucket
-	11, // 14: opencloud.services.search.v0.AggregationResult.metric:type_name -> opencloud.services.search.v0.Metric
-	0,  // 15: opencloud.services.search.v0.Metric.kind:type_name -> opencloud.services.search.v0.MetricKind
-	17, // 16: opencloud.services.search.v0.IndexSpaceResponse.space_duration:type_name -> google.protobuf.Duration
-	2,  // 17: opencloud.services.search.v0.SearchProvider.Search:input_type -> opencloud.services.search.v0.SearchRequest
-	13, // 18: opencloud.services.search.v0.SearchProvider.IndexSpace:input_type -> opencloud.services.search.v0.IndexSpaceRequest
-	4,  // 19: opencloud.services.search.v0.IndexProvider.Search:input_type -> opencloud.services.search.v0.SearchIndexRequest
-	3,  // 20: opencloud.services.search.v0.SearchProvider.Search:output_type -> opencloud.services.search.v0.SearchResponse
-	14, // 21: opencloud.services.search.v0.SearchProvider.IndexSpace:output_type -> opencloud.services.search.v0.IndexSpaceResponse
-	5,  // 22: opencloud.services.search.v0.IndexProvider.Search:output_type -> opencloud.services.search.v0.SearchIndexResponse
-	20, // [20:23] is the sub-list for method output_type
-	17, // [17:20] is the sub-list for method input_type
-	17, // [17:17] is the sub-list for extension type_name
-	17, // [17:17] is the sub-list for extension extendee
-	0,  // [0:17] is the sub-list for field type_name
+	6,  // 10: opencloud.services.search.v0.AggregationOption.sub_aggregations:type_name -> opencloud.services.search.v0.AggregationOption
+	0,  // 11: opencloud.services.search.v0.MetricDefinition.kind:type_name -> opencloud.services.search.v0.MetricKind
+	1,  // 12: opencloud.services.search.v0.BucketDefinition.sort_by:type_name -> opencloud.services.search.v0.BucketSortBy
+	9,  // 13: opencloud.services.search.v0.BucketDefinition.ranges:type_name -> opencloud.services.search.v0.BucketRange
+	12, // 14: opencloud.services.search.v0.AggregationResult.buckets:type_name -> opencloud.services.search.v0.Bucket
+	11, // 15: opencloud.services.search.v0.AggregationResult.metric:type_name -> opencloud.services.search.v0.Metric
+	0,  // 16: opencloud.services.search.v0.Metric.kind:type_name -> opencloud.services.search.v0.MetricKind
+	10, // 17: opencloud.services.search.v0.Bucket.sub_aggregations:type_name -> opencloud.services.search.v0.AggregationResult
+	17, // 18: opencloud.services.search.v0.IndexSpaceResponse.space_duration:type_name -> google.protobuf.Duration
+	2,  // 19: opencloud.services.search.v0.SearchProvider.Search:input_type -> opencloud.services.search.v0.SearchRequest
+	13, // 20: opencloud.services.search.v0.SearchProvider.IndexSpace:input_type -> opencloud.services.search.v0.IndexSpaceRequest
+	4,  // 21: opencloud.services.search.v0.IndexProvider.Search:input_type -> opencloud.services.search.v0.SearchIndexRequest
+	3,  // 22: opencloud.services.search.v0.SearchProvider.Search:output_type -> opencloud.services.search.v0.SearchResponse
+	14, // 23: opencloud.services.search.v0.SearchProvider.IndexSpace:output_type -> opencloud.services.search.v0.IndexSpaceResponse
+	5,  // 24: opencloud.services.search.v0.IndexProvider.Search:output_type -> opencloud.services.search.v0.SearchIndexResponse
+	22, // [22:25] is the sub-list for method output_type
+	19, // [19:22] is the sub-list for method input_type
+	19, // [19:19] is the sub-list for extension type_name
+	19, // [19:19] is the sub-list for extension extendee
+	0,  // [0:19] is the sub-list for field type_name
 }
 
 func init() { file_opencloud_services_search_v0_search_proto_init() }
diff --git a/protogen/gen/opencloud/services/search/v0/search.swagger.json b/protogen/gen/opencloud/services/search/v0/search.swagger.json
index 50f52d2b7b..d932e3c7da 100644
--- a/protogen/gen/opencloud/services/search/v0/search.swagger.json
+++ b/protogen/gen/opencloud/services/search/v0/search.swagger.json
@@ -186,6 +186,13 @@
         "metricDefinition": {
           "$ref": "#/definitions/v0MetricDefinition",
           "description": "Optional. When set, this aggregation is a scalar metric over `field`\nrather than a bucket aggregation; the corresponding AggregationResult\ncarries `metric` instead of `buckets`."
+        },
+        "subAggregations": {
+          "type": "array",
+          "items": {
+            "$ref": "#/definitions/v0AggregationOption"
+          },
+          "description": "Optional. Nested aggregations computed within each bucket of this\naggregation."
         }
       }
     },
@@ -277,6 +284,13 @@
         "count": {
           "type": "string",
           "format": "int64"
+        },
+        "subAggregations": {
+          "type": "array",
+          "items": {
+            "$ref": "#/definitions/v0AggregationResult"
+          },
+          "description": "Nested aggregation results, one per sub_aggregation of the parent\nAggregationOption, at the same position."
         }
       }
     },
diff --git a/protogen/proto/opencloud/services/search/v0/search.proto b/protogen/proto/opencloud/services/search/v0/search.proto
index 5f81142fdf..2bc2ff12cf 100644
--- a/protogen/proto/opencloud/services/search/v0/search.proto
+++ b/protogen/proto/opencloud/services/search/v0/search.proto
@@ -138,6 +138,9 @@ message AggregationOption {
   // rather than a bucket aggregation; the corresponding AggregationResult
   // carries `metric` instead of `buckets`.
   MetricDefinition metric_definition = 4 [(google.api.field_behavior) = OPTIONAL];
+  // Optional. Nested aggregations computed within each bucket of this
+  // aggregation.
+  repeated AggregationOption sub_aggregations = 5 [(google.api.field_behavior) = OPTIONAL];
 }
 
 message MetricDefinition {
@@ -205,6 +208,9 @@ message Metric {
 message Bucket {
   string key = 1;
   int64 count = 2;
+  // Nested aggregation results, one per sub_aggregation of the parent
+  // AggregationOption, at the same position.
+  repeated AggregationResult sub_aggregations = 3;
 }
 
 message IndexSpaceRequest {
diff --git a/services/graph/pkg/service/v0/searchquery.go b/services/graph/pkg/service/v0/searchquery.go
index be86393775..a899b52393 100644
--- a/services/graph/pkg/service/v0/searchquery.go
+++ b/services/graph/pkg/service/v0/searchquery.go
@@ -92,13 +92,17 @@ func unsupportedProperty(sr libregraph.SearchRequest) string {
 	if len(sr.AggregationFilters) > 0 {
 		return "aggregationFilters"
 	}
-	for _, a := range sr.Aggregations {
-		switch {
-		case a.LibreGraphGeohashDefinition != nil:
-			return "@libre.graph.geohashDefinition"
-		case len(a.LibreGraphSubAggregations) > 0:
-			return "@libre.graph.subAggregations"
+	var geohash func(aggs []libregraph.AggregationOption) bool
+	geohash = func(aggs []libregraph.AggregationOption) bool {
+		for _, a := range aggs {
+			if a.LibreGraphGeohashDefinition != nil || geohash(a.LibreGraphSubAggregations) {
+				return true
+			}
 		}
+		return false
+	}
+	if geohash(sr.Aggregations) {
+		return "@libre.graph.geohashDefinition"
 	}
 	return ""
 }
@@ -243,6 +247,9 @@ func validateAggregations(aggs []libregraph.AggregationOption) error {
 				return fmt.Errorf("minimumCount of the aggregation on %q must not be negative", a.Field)
 			}
 		}
+		if err := validateAggregations(a.LibreGraphSubAggregations); err != nil {
+			return err
+		}
 	}
 	return nil
 }
diff --git a/services/graph/pkg/service/v0/searchquery_mapping.go b/services/graph/pkg/service/v0/searchquery_mapping.go
index 1a944ec830..d5344fbf42 100644
--- a/services/graph/pkg/service/v0/searchquery_mapping.go
+++ b/services/graph/pkg/service/v0/searchquery_mapping.go
@@ -55,6 +55,7 @@ func indexField(field string) string {
 func resolveFields(opts []*searchsvc.AggregationOption) {
 	for _, opt := range opts {
 		opt.Field = indexField(opt.Field)
+		resolveFields(opt.SubAggregations)
 	}
 }
 
@@ -68,6 +69,9 @@ func libregraphAggregationsToSearch(in []libregraph.AggregationOption) []*search
 		if a.BucketDefinition != nil {
 			agg.BucketDefinition = libregraphBucketDefinitionToSearch(*a.BucketDefinition)
 		}
+		if len(a.LibreGraphSubAggregations) > 0 {
+			agg.SubAggregations = libregraphAggregationsToSearch(a.LibreGraphSubAggregations)
+		}
 		if md := a.LibreGraphMetricDefinition; md != nil {
 			agg.MetricDefinition = &searchsvc.MetricDefinition{Kind: metricKinds[md.Kind]}
 		}
@@ -99,8 +103,8 @@ func libregraphRangesToSearch(in []libregraph.BucketAggregationRange) []*searchs
 }
 
 // searchAggregationsToLibregraph maps the results to their definitions by
-// position: the search service answers one result per aggregation, in request
-// order.
+// position, at every level: the search service answers one result per
+// aggregation, in request order.
 func searchAggregationsToLibregraph(in []*searchsvc.AggregationResult, defs []libregraph.AggregationOption) []libregraph.SearchAggregation {
 	if len(in) == 0 {
 		return nil
@@ -123,8 +127,9 @@ func searchAggregationsToLibregraph(in []*searchsvc.AggregationResult, defs []li
 		agg.Buckets = make([]libregraph.SearchBucket, 0, len(in[i].GetBuckets()))
 		for _, b := range in[i].GetBuckets() {
 			agg.Buckets = append(agg.Buckets, libregraph.SearchBucket{
-				Key:   libregraph.PtrString(b.GetKey()),
-				Count: libregraph.PtrInt64(b.GetCount()),
+				Key:                       libregraph.PtrString(b.GetKey()),
+				Count:                     libregraph.PtrInt64(b.GetCount()),
+				LibreGraphSubAggregations: searchAggregationsToLibregraph(b.GetSubAggregations(), def.LibreGraphSubAggregations),
 			})
 		}
 		out = append(out, agg)
diff --git a/services/graph/pkg/service/v0/searchquery_test.go b/services/graph/pkg/service/v0/searchquery_test.go
index 0ca52122cc..ae94c90fb5 100644
--- a/services/graph/pkg/service/v0/searchquery_test.go
+++ b/services/graph/pkg/service/v0/searchquery_test.go
@@ -92,8 +92,9 @@ type searchAggregationJSON struct {
 		Value *float64 `json:"value"`
 	} `json:"@libre.graph.metric"`
 	Buckets []struct {
-		Key   string `json:"key"`
-		Count int64  `json:"count"`
+		Key             string                  `json:"key"`
+		Count           int64                   `json:"count"`
+		SubAggregations []searchAggregationJSON `json:"@libre.graph.subAggregations"`
 	} `json:"buckets"`
 }
 
@@ -309,6 +310,8 @@ var _ = ginkgo.Describe("SearchQuery", func() {
 		ginkgo.Entry("an unknown sortBy", `"aggregations": [{"field": "audio.artist", "bucketDefinition": {"sortBy": "relevance"}}]`),
 		ginkgo.Entry("a size below one", `"aggregations": [{"field": "audio.artist", "size": 0}]`),
 		ginkgo.Entry("a negative minimumCount", `"aggregations": [{"field": "audio.artist", "bucketDefinition": {"sortBy": "count", "minimumCount": -1}}]`),
+		ginkgo.Entry("an unknown metric kind two levels down",
+			`"aggregations": [{"field": "audio.artist", "@libre.graph.subAggregations": [{"field": "audio.album", "@libre.graph.subAggregations": [{"field": "audio.year", "@libre.graph.metricDefinition": {"kind": "median"}}]}]}]`),
 		ginkgo.Entry("a field the index does not know", `"aggregations": [{"field": "audio.nonexistent"}]`),
 		ginkgo.Entry("a range bound that is no number", `"aggregations": [{"field": "audio.year", "bucketDefinition": {"sortBy": "count", "ranges": [{"from": "1970", "to": "198o"}]}}]`),
 	)
@@ -407,7 +410,8 @@ var _ = ginkgo.Describe("SearchQuery", func() {
 		ginkgo.Entry("aggregationFilters", `"aggregationFilters": ["audio.artist:\"ǂǂ5361786f6e\""]`, "aggregationFilters"),
 		ginkgo.Entry("a geohash aggregation",
 			`"aggregations": [{"field": "location", "@libre.graph.geohashDefinition": {"precision": 5}}]`, "geohashDefinition"),
-		ginkgo.Entry("sub-aggregations", `"aggregations": [{"field": "audio.artist", "@libre.graph.subAggregations": [{"field": "audio.album"}]}]`, "subAggregations"),
+		ginkgo.Entry("a nested geohash aggregation",
+			`"aggregations": [{"field": "audio.artist", "@libre.graph.subAggregations": [{"field": "location", "@libre.graph.geohashDefinition": {"precision": 5}}]}]`, "geohashDefinition"),
 	)
 
 	ginkgo.It("rejects an $expand it does not know with 400", func() {
diff --git a/services/search/pkg/aggregation/aggregation.go b/services/search/pkg/aggregation/aggregation.go
index e1dd3d4936..61cc9c0602 100644
--- a/services/search/pkg/aggregation/aggregation.go
+++ b/services/search/pkg/aggregation/aggregation.go
@@ -32,8 +32,8 @@ func KindOf(opt *searchsvc.AggregationOption) Kind {
 }
 
 // MaxBuckets is how many buckets the aggregations of one request may have in
-// one space: the search.max_buckets default of OpenSearch, held on every
-// engine.
+// one space, all levels counted: the search.max_buckets default of OpenSearch,
+// held on every engine.
 const MaxBuckets = math.MaxUint16
 
 // ErrTooManyBuckets is what an engine answers beyond MaxBuckets; the service
@@ -52,6 +52,9 @@ func countBuckets(results []*searchsvc.AggregationResult) int {
 	n := 0
 	for _, r := range results {
 		n += len(r.GetBuckets())
+		for _, b := range r.GetBuckets() {
+			n += countBuckets(b.GetSubAggregations())
+		}
 	}
 	return n
 }
diff --git a/services/search/pkg/aggregation/aggregation_test.go b/services/search/pkg/aggregation/aggregation_test.go
index d5746314ce..7a2b94e861 100644
--- a/services/search/pkg/aggregation/aggregation_test.go
+++ b/services/search/pkg/aggregation/aggregation_test.go
@@ -11,6 +11,17 @@ import (
 )
 
 var _ = Describe("CheckBuckets", func() {
+	// artists of an album each: twice as many buckets as artists
+	artistsWithAlbum := func(artists int) []*searchsvc.AggregationResult {
+		res := &searchsvc.AggregationResult{Field: "audio.artist"}
+		for i := range artists {
+			res.Buckets = append(res.Buckets, &searchsvc.Bucket{Key: strconv.Itoa(i), Count: 1, SubAggregations: []*searchsvc.AggregationResult{
+				{Field: "audio.album", Buckets: []*searchsvc.Bucket{{Key: "Singles", Count: 1}}},
+			}})
+		}
+		return []*searchsvc.AggregationResult{res}
+	}
+
 	artists := func(count int) []*searchsvc.AggregationResult {
 		res := &searchsvc.AggregationResult{Field: "audio.artist"}
 		for i := range count {
@@ -25,4 +36,9 @@ var _ = Describe("CheckBuckets", func() {
 		Expect(aggregation.CheckBuckets(artists(65535))).To(Succeed())
 		Expect(aggregation.CheckBuckets(artists(65536))).To(MatchError(aggregation.ErrTooManyBuckets))
 	})
+
+	It("counts the buckets of all levels", func() {
+		Expect(aggregation.CheckBuckets(artistsWithAlbum(32767))).To(Succeed(), "65534 buckets")
+		Expect(aggregation.CheckBuckets(artistsWithAlbum(32768))).To(MatchError(aggregation.ErrTooManyBuckets), "65536 buckets")
+	})
 })
diff --git a/services/search/pkg/aggregation/merge.go b/services/search/pkg/aggregation/merge.go
index ecbd584f93..9ba975a333 100644
--- a/services/search/pkg/aggregation/merge.go
+++ b/services/search/pkg/aggregation/merge.go
@@ -29,14 +29,15 @@ func Empty(opts []*searchsvc.AggregationOption) []*searchsvc.AggregationResult {
 }
 
 // RangeBuckets lists every range of a range aggregation in request order: the
-// answered bucket where there is one, a count of zero otherwise.
+// answered bucket where there is one, a count of zero and empty
+// sub-aggregations otherwise.
 func RangeBuckets(opt *searchsvc.AggregationOption, answered map[string]*searchsvc.Bucket) []*searchsvc.Bucket {
 	ranges := opt.GetBucketDefinition().GetRanges()
 	out := make([]*searchsvc.Bucket, 0, len(ranges))
 	for _, r := range ranges {
 		bucket, ok := answered[RangeKey(r)]
 		if !ok {
-			bucket = &searchsvc.Bucket{Key: RangeKey(r)}
+			bucket = &searchsvc.Bucket{Key: RangeKey(r), SubAggregations: Empty(opt.GetSubAggregations())}
 		}
 		out = append(out, bucket)
 	}
@@ -44,8 +45,8 @@ func RangeBuckets(opt *searchsvc.AggregationOption, answered map[string]*searchs
 }
 
 // Merge folds the results of one space into acc and returns it. Results
-// belong to the option at their position, so several aggregations on one
-// field stay apart.
+// belong to the option at their position, at every level, so several
+// aggregations on one field stay apart.
 func Merge(opts []*searchsvc.AggregationOption, acc, results []*searchsvc.AggregationResult) []*searchsvc.AggregationResult {
 	if acc == nil {
 		acc = Empty(opts)
@@ -70,13 +71,16 @@ func Merge(opts []*searchsvc.AggregationOption, acc, results []*searchsvc.Aggreg
 				acc[i].Buckets = append(acc[i].Buckets, merged)
 			}
 			merged.Count += b.GetCount()
+			if subs := opt.GetSubAggregations(); len(subs) > 0 {
+				merged.SubAggregations = Merge(subs, merged.GetSubAggregations(), b.GetSubAggregations())
+			}
 		}
 	}
 	return acc
 }
 
-// Finalize shapes merged results per their options: buckets get their minimum
-// count, order and size, metrics their value.
+// Finalize shapes merged results per their options, at every level: buckets
+// get their minimum count, order and size, metrics their value.
 func Finalize(opts []*searchsvc.AggregationOption, results []*searchsvc.AggregationResult) {
 	for i, opt := range opts {
 		r := results[i]
@@ -87,6 +91,9 @@ func Finalize(opts []*searchsvc.AggregationOption, results []*searchsvc.Aggregat
 			continue
 		}
 		r.Buckets = shapeBuckets(opt, r.GetBuckets())
+		for _, b := range r.GetBuckets() {
+			Finalize(opt.GetSubAggregations(), b.GetSubAggregations())
+		}
 	}
 }
 
diff --git a/services/search/pkg/aggregation/merge_test.go b/services/search/pkg/aggregation/merge_test.go
index 35c4b2fe99..7cea3019be 100644
--- a/services/search/pkg/aggregation/merge_test.go
+++ b/services/search/pkg/aggregation/merge_test.go
@@ -31,8 +31,8 @@ func buckets(field string, bs ...*searchsvc.Bucket) *searchsvc.AggregationResult
 	return &searchsvc.AggregationResult{Field: field, Buckets: bs}
 }
 
-func bucket(key string, count int64) *searchsvc.Bucket {
-	return &searchsvc.Bucket{Key: key, Count: count}
+func bucket(key string, count int64, subs ...*searchsvc.AggregationResult) *searchsvc.Bucket {
+	return &searchsvc.Bucket{Key: key, Count: count, SubAggregations: subs}
 }
 
 func rangesOf(sortBy searchsvc.BucketSortBy, rs ...*searchsvc.BucketRange) *searchsvc.BucketDefinition {
@@ -40,23 +40,29 @@ func rangesOf(sortBy searchsvc.BucketSortBy, rs ...*searchsvc.BucketRange) *sear
 }
 
 // render flattens results like the parity suite: one line per bucket or
-// metric.
+// metric, nested ones below their bucket.
 func render(results []*searchsvc.AggregationResult) []string {
-	out := []string{}
-	for _, r := range results {
-		if m := r.GetMetric(); m != nil {
-			if m.Value == nil {
-				out = append(out, fmt.Sprintf("%s %s none", r.GetField(), m.GetKind()))
-			} else {
-				out = append(out, fmt.Sprintf("%s %s=%v", r.GetField(), m.GetKind(), m.GetValue()))
+	var walk func(prefix string, results []*searchsvc.AggregationResult) []string
+	walk = func(prefix string, results []*searchsvc.AggregationResult) []string {
+		out := []string{}
+		for _, r := range results {
+			if m := r.GetMetric(); m != nil {
+				if m.Value == nil {
+					out = append(out, prefix+fmt.Sprintf("%s %s none", r.GetField(), m.GetKind()))
+				} else {
+					out = append(out, prefix+fmt.Sprintf("%s %s=%v", r.GetField(), m.GetKind(), m.GetValue()))
+				}
+				continue
+			}
+			for _, b := range r.GetBuckets() {
+				line := prefix + fmt.Sprintf("%s %s=%d", r.GetField(), b.GetKey(), b.GetCount())
+				out = append(out, line)
+				out = append(out, walk(line+" / ", b.GetSubAggregations())...)
 			}
-			continue
-		}
-		for _, b := range r.GetBuckets() {
-			out = append(out, fmt.Sprintf("%s %s=%d", r.GetField(), b.GetKey(), b.GetCount()))
 		}
+		return out
 	}
-	return out
+	return walk("", results)
 }
 
 // merged runs the service layer's fold over the answers of several spaces.
@@ -81,7 +87,8 @@ var _ = Describe("Merge and Finalize", func() {
 		opts := []*searchsvc.AggregationOption{
 			{Field: "audio.artist"},
 			metricOpt("audio.year", sum),
-			{Field: "audio.year", BucketDefinition: rangesOf(searchsvc.BucketSortBy_BUCKET_SORT_BY_KEY_AS_NUMBER, &searchsvc.BucketRange{To: "1980"}, &searchsvc.BucketRange{From: "1980"})},
+			{Field: "audio.year", BucketDefinition: rangesOf(searchsvc.BucketSortBy_BUCKET_SORT_BY_KEY_AS_NUMBER, &searchsvc.BucketRange{To: "1980"}, &searchsvc.BucketRange{From: "1980"}),
+				SubAggregations: []*searchsvc.AggregationOption{{Field: "audio.artist"}, metricOpt("audio.year", max)}},
 		}
 		got := merged(opts)
 		Expect(got).To(HaveLen(3))
@@ -89,7 +96,8 @@ var _ = Describe("Merge and Finalize", func() {
 		Expect(got[0].GetBuckets()).To(BeEmpty())
 		Expect(render(got)).To(Equal([]string{
 			"audio.year METRIC_KIND_SUM none",
-			"audio.year ..1980=0", "audio.year 1980..=0",
+			"audio.year ..1980=0", "audio.year ..1980=0 / audio.year METRIC_KIND_MAX none",
+			"audio.year 1980..=0", "audio.year 1980..=0 / audio.year METRIC_KIND_MAX none",
 		}))
 	})
 
@@ -146,6 +154,44 @@ var _ = Describe("Merge and Finalize", func() {
 		}))
 	})
 
+	It("keeps sibling metrics on one field apart below a bucket that spans spaces", func() {
+		opts := []*searchsvc.AggregationOption{{Field: "audio.artist", SubAggregations: []*searchsvc.AggregationOption{
+			metricOpt("audio.year", sum),
+			metricOpt("audio.year", avg),
+		}}}
+		space := func(years ...float64) []*searchsvc.AggregationResult {
+			return []*searchsvc.AggregationResult{buckets("audio.artist",
+				bucket("Saxon", int64(len(years)), metric("audio.year", sum, years...), metric("audio.year", avg, years...)),
+			)}
+		}
+		got := merged(opts, space(1971, 1975), space(1980))
+		Expect(render(got)).To(Equal([]string{
+			"audio.artist Saxon=3",
+			"audio.artist Saxon=3 / audio.year METRIC_KIND_SUM=5926",
+			"audio.artist Saxon=3 / audio.year METRIC_KIND_AVG=1975.3333333333333",
+		}))
+	})
+
+	It("answers nested results in request order", func() {
+		opts := []*searchsvc.AggregationOption{{Field: "audio.artist", SubAggregations: []*searchsvc.AggregationOption{
+			{Field: "audio.genre"}, {Field: "audio.album"}, metricOpt("audio.year", max), {Field: "audio.composers"},
+		}}}
+		space := []*searchsvc.AggregationResult{buckets("audio.artist", bucket("Saxon", 1,
+			buckets("audio.genre", bucket("Metal", 1)),
+			buckets("audio.album", bucket("Wheels of Steel", 1)),
+			metric("audio.year", max, 1980),
+			buckets("audio.composers", bucket("Byford", 1)),
+		))}
+		for range 20 {
+			got := merged(opts, space, space)
+			fields := []string{}
+			for _, sub := range got[0].GetBuckets()[0].GetSubAggregations() {
+				fields = append(fields, sub.GetField())
+			}
+			Expect(fields).To(Equal([]string{"audio.genre", "audio.album", "audio.year", "audio.composers"}))
+		}
+	})
+
 	DescribeTable("reduces a metric over the values of all spaces",
 		func(kind searchsvc.MetricKind, want string) {
 			opts := []*searchsvc.AggregationOption{metricOpt("audio.year", kind)}
@@ -237,5 +283,21 @@ var _ = Describe("Merge and Finalize", func() {
 			)
 			Expect(render(got)).To(Equal([]string{"audio.artist Saxon=2"}))
 		})
+
+		It("shapes the buckets of a sub-aggregation by its own definition", func() {
+			opts := []*searchsvc.AggregationOption{{Field: "audio.artist", SubAggregations: []*searchsvc.AggregationOption{{
+				Field: "audio.album", Size: 2,
+				BucketDefinition: &searchsvc.BucketDefinition{SortBy: searchsvc.BucketSortBy_BUCKET_SORT_BY_KEY_AS_STRING, MinimumCount: 2},
+			}}}}
+			space := []*searchsvc.AggregationResult{buckets("audio.artist", bucket("Saxon", 4, buckets("audio.album",
+				bucket("Wheels of Steel", 1), bucket("Strong Arm of the Law", 1), bucket("Denim and Leather", 1), bucket("Crusader", 1),
+			)))}
+			Expect(render(merged(opts, space, space))).To(Equal([]string{
+				"audio.artist Saxon=8",
+				"audio.artist Saxon=8 / audio.album Crusader=2",
+				"audio.artist Saxon=8 / audio.album Denim and Leather=2",
+			}))
+			Expect(render(merged(opts, space))).To(Equal([]string{"audio.artist Saxon=4"}), "one space alone stays below the minimum count")
+		})
 	})
 })
diff --git a/services/search/pkg/aggregation/validate.go b/services/search/pkg/aggregation/validate.go
index d79e90505a..14d72e1035 100644
--- a/services/search/pkg/aggregation/validate.go
+++ b/services/search/pkg/aggregation/validate.go
@@ -21,6 +21,9 @@ func ValidateOptions(opts []*searchsvc.AggregationOption, fieldType func(string)
 			if t != mapping.TypeNumeric {
 				return fmt.Errorf("metric aggregation needs a numeric field, %q is none", field)
 			}
+			if len(opt.GetSubAggregations()) > 0 {
+				return fmt.Errorf("metric aggregation on %q has no buckets to nest sub-aggregations in", field)
+			}
 		case KindRange:
 			parsed, err := ParseRanges(field, opt.GetBucketDefinition().GetRanges())
 			if err != nil {
@@ -34,6 +37,9 @@ func ValidateOptions(opts []*searchsvc.AggregationOption, fieldType func(string)
 				return err
 			}
 		}
+		if err := ValidateOptions(opt.GetSubAggregations(), fieldType); err != nil {
+			return err
+		}
 	}
 	return nil
 }
diff --git a/services/search/pkg/aggregation/validate_test.go b/services/search/pkg/aggregation/validate_test.go
index 0fd28c59c3..762180e0d9 100644
--- a/services/search/pkg/aggregation/validate_test.go
+++ b/services/search/pkg/aggregation/validate_test.go
@@ -39,6 +39,11 @@ var _ = Describe("ValidateOptions", func() {
 		Entry("numeric ranges on a number", &searchsvc.AggregationOption{Field: "audio.year", BucketDefinition: numeric}),
 		Entry("date ranges on a date", &searchsvc.AggregationOption{Field: "photo.takenDateTime", BucketDefinition: dates}),
 		Entry("a metric on a number", &searchsvc.AggregationOption{Field: "audio.year", MetricDefinition: sum}),
+		Entry("a tree of them", &searchsvc.AggregationOption{Field: "audio.artist", SubAggregations: []*searchsvc.AggregationOption{
+			{Field: "audio.year", BucketDefinition: numeric, SubAggregations: []*searchsvc.AggregationOption{
+				{Field: "audio.album"}, {Field: "audio.year", MetricDefinition: sum},
+			}},
+		}}),
 	)
 
 	DescribeTable("rejects an aggregation the index cannot answer",
@@ -53,8 +58,13 @@ var _ = Describe("ValidateOptions", func() {
 		Entry("terms on a geopoint", &searchsvc.AggregationOption{Field: "location"}),
 		Entry("a metric on a keyword", &searchsvc.AggregationOption{Field: "audio.artist", MetricDefinition: sum}),
 		Entry("a metric on a date", &searchsvc.AggregationOption{Field: "photo.takenDateTime", MetricDefinition: sum}),
+		Entry("a metric with sub-aggregations", &searchsvc.AggregationOption{Field: "audio.year", MetricDefinition: sum,
+			SubAggregations: []*searchsvc.AggregationOption{{Field: "audio.artist"}}}),
 		Entry("numeric ranges on a keyword", &searchsvc.AggregationOption{Field: "audio.artist", BucketDefinition: numeric}),
 		Entry("numeric ranges on a date", &searchsvc.AggregationOption{Field: "photo.takenDateTime", BucketDefinition: numeric}),
 		Entry("date ranges on a number", &searchsvc.AggregationOption{Field: "audio.year", BucketDefinition: dates}),
+		Entry("an invalid aggregation two levels down", &searchsvc.AggregationOption{Field: "audio.artist", SubAggregations: []*searchsvc.AggregationOption{
+			{Field: "audio.album", SubAggregations: []*searchsvc.AggregationOption{{Field: "photo.takenDateTime"}}},
+		}}),
 	)
 })
diff --git a/services/search/pkg/bleve/aggregations.go b/services/search/pkg/bleve/aggregations.go
index 503c6e5ffb..f4948cb856 100644
--- a/services/search/pkg/bleve/aggregations.go
+++ b/services/search/pkg/bleve/aggregations.go
@@ -17,8 +17,8 @@ import (
 )
 
 // Bleve facets count the indexed terms of one field (numbers among them as
-// prefix-coded terms), so every aggregation is folded from doc values by
-// aggCollector, hooked into the collector walk through bleve's
+// prefix-coded terms) and cannot nest, so every aggregation is folded from doc
+// values by aggCollector, hooked into the collector walk through bleve's
 // document-match-handler context key. Loading hits for them instead costs a
 // stored-document decode per match.
 
@@ -39,6 +39,7 @@ type aggLevel struct {
 	kind      aggregation.Kind
 	fieldType string
 	ranges    aggregation.Ranges
+	children  []*aggLevel
 }
 
 // numeric values are prefix-coded in the index, their terms come from the
@@ -56,6 +57,13 @@ func newAggLevel(opt *searchService.AggregationOption) (*aggLevel, error) {
 		}
 		l.ranges = ranges
 	}
+	for _, sub := range opt.GetSubAggregations() {
+		child, err := newAggLevel(sub)
+		if err != nil {
+			return nil, err
+		}
+		l.children = append(l.children, child)
+	}
 	return l, nil
 }
 
@@ -69,6 +77,7 @@ type fieldValues struct {
 
 type bucketAcc struct {
 	counts map[string]int64
+	subs   map[string][]*bucketAcc
 	metric *searchService.Metric
 }
 
@@ -76,7 +85,11 @@ func newBucketAcc(l *aggLevel) *bucketAcc {
 	if l.kind == aggregation.KindMetric {
 		return &bucketAcc{metric: &searchService.Metric{Kind: l.opt.GetMetricDefinition().GetKind()}}
 	}
-	return &bucketAcc{counts: map[string]int64{}}
+	a := &bucketAcc{counts: map[string]int64{}}
+	if len(l.children) > 0 {
+		a.subs = map[string][]*bucketAcc{}
+	}
+	return a
 }
 
 // aggCollector serves one search; bleve's collector is single-threaded.
@@ -119,6 +132,9 @@ func (c *aggCollector) register(l *aggLevel) {
 	} else {
 		fv.asNumbers = true
 	}
+	for _, child := range l.children {
+		c.register(child)
+	}
 }
 
 // The handler runs for every match, before the top-n cut.
@@ -216,8 +232,22 @@ func (c *aggCollector) fold(a *bucketAcc, l *aggLevel) {
 	}
 }
 
-func (c *aggCollector) foldBucket(a *bucketAcc, _ *aggLevel, key string) {
+func (c *aggCollector) foldBucket(a *bucketAcc, l *aggLevel, key string) {
 	a.counts[key]++
+	if len(l.children) == 0 {
+		return
+	}
+	subs, ok := a.subs[key]
+	if !ok {
+		subs = make([]*bucketAcc, len(l.children))
+		for i, child := range l.children {
+			subs[i] = newBucketAcc(child)
+		}
+		a.subs[key] = subs
+	}
+	for i, child := range l.children {
+		c.fold(subs[i], child)
+	}
 }
 
 // results returns one result per aggregation, in request order.
@@ -238,7 +268,11 @@ func (a *bucketAcc) result(l *aggLevel) *searchService.AggregationResult {
 
 	counted := make(map[string]*searchService.Bucket, len(a.counts))
 	for key, count := range a.counts {
-		counted[key] = &searchService.Bucket{Key: key, Count: count}
+		b := &searchService.Bucket{Key: key, Count: count}
+		for i, child := range l.children {
+			b.SubAggregations = append(b.SubAggregations, a.subs[key][i].result(child))
+		}
+		counted[key] = b
 	}
 	if l.kind == aggregation.KindRange {
 		r.Buckets = aggregation.RangeBuckets(l.opt, counted)
diff --git a/services/search/pkg/opensearch/internal/aggs/aggs.go b/services/search/pkg/opensearch/internal/aggs/aggs.go
index fc0217fad8..5d2df9c6f0 100644
--- a/services/search/pkg/opensearch/internal/aggs/aggs.go
+++ b/services/search/pkg/opensearch/internal/aggs/aggs.go
@@ -12,10 +12,10 @@ import (
 	"github.com/opencloud-eu/opencloud/services/search/pkg/aggregation"
 )
 
-// Build renders the aggregations as native OpenSearch aggregations, each
-// named by its position. A terms aggregation asks for every bucket up to the
-// limit: the service layer cuts to the requested size after the cross-space
-// merge, a cut here would make that merge inexact.
+// Build renders the aggregation tree as native OpenSearch aggregations, each
+// named by its position among its siblings. A terms aggregation asks for
+// every bucket up to the limit: the service layer cuts to the requested size
+// after the cross-space merge, a cut here would make that merge inexact.
 func Build(opts []*searchsvc.AggregationOption) (map[string]any, error) {
 	if len(opts) == 0 {
 		return nil, nil
@@ -51,6 +51,13 @@ func build(opt *searchsvc.AggregationOption) (map[string]any, error) {
 	default:
 		entry = map[string]any{"terms": map[string]any{"field": opt.GetField(), "size": aggregation.MaxBuckets}}
 	}
+	children, err := Build(opt.GetSubAggregations())
+	if err != nil {
+		return nil, err
+	}
+	if children != nil {
+		entry["aggs"] = children
+	}
 	return entry, nil
 }
 
@@ -159,10 +166,18 @@ func parseBucket(opt *searchsvc.AggregationOption, raw json.RawMessage) (*search
 		KeyAsString *string `json:"key_as_string"`
 		DocCount    int64   `json:"doc_count"`
 	}
+	children := aggNode{}
 	if err := json.Unmarshal(raw, &head); err != nil {
 		return nil, fmt.Errorf("decode bucket of %s: %w", opt.GetField(), err)
 	}
-	return &searchsvc.Bucket{Key: bucketKey(head.Key, head.KeyAsString), Count: head.DocCount}, nil
+	if err := json.Unmarshal(raw, &children); err != nil {
+		return nil, fmt.Errorf("decode bucket of %s: %w", opt.GetField(), err)
+	}
+	subs, err := parse(opt.GetSubAggregations(), children)
+	if err != nil {
+		return nil, err
+	}
+	return &searchsvc.Bucket{Key: bucketKey(head.Key, head.KeyAsString), Count: head.DocCount, SubAggregations: subs}, nil
 }
 
 // bucketKey spells a response key like the bleve backend does: a number
diff --git a/services/search/pkg/opensearch/internal/aggs/aggs_test.go b/services/search/pkg/opensearch/internal/aggs/aggs_test.go
index e333418e2d..6e60b22086 100644
--- a/services/search/pkg/opensearch/internal/aggs/aggs_test.go
+++ b/services/search/pkg/opensearch/internal/aggs/aggs_test.go
@@ -110,6 +110,11 @@ var _ = Describe("Aggregations", func() {
 		Expect(err).To(HaveOccurred())
 	})
 
+	It("rejects a malformed range below another aggregation", func() {
+		_, err := aggs.Build([]*searchsvc.AggregationOption{{Field: "audio.artist", SubAggregations: []*searchsvc.AggregationOption{malformed}}})
+		Expect(err).To(HaveOccurred())
+	})
+
 	It("lists every requested range in request order, empty ones included", func() {
 		out := parse(`{"a_0": {"buckets": [
 			{"key": "1970..1980", "doc_count": 4},
@@ -177,4 +182,58 @@ var _ = Describe("Aggregations", func() {
 			&searchsvc.AggregationOption{Field: "audio.year", MetricDefinition: &searchsvc.MetricDefinition{Kind: searchsvc.MetricKind_METRIC_KIND_SUM}},
 			`{"a_0": {"count": "many"}}`),
 	)
+
+	It("nests sub-aggregations in their parent, named by position", func() {
+		artist := build(&searchsvc.AggregationOption{
+			Field: "audio.artist",
+			SubAggregations: []*searchsvc.AggregationOption{
+				{Field: "audio.year", BucketDefinition: &searchsvc.BucketDefinition{Ranges: []*searchsvc.BucketRange{{To: "1980"}}},
+					SubAggregations: []*searchsvc.AggregationOption{{Field: "audio.album"}}},
+				{Field: "audio.duration", MetricDefinition: &searchsvc.MetricDefinition{Kind: searchsvc.MetricKind_METRIC_KIND_SUM}},
+			},
+		})["a_0"].(map[string]any)
+		children := artist["aggs"].(map[string]any)
+		Expect(children).To(HaveKey("a_1"), "the metric")
+		Expect(children["a_1"].(map[string]any)).To(HaveKey("stats"))
+
+		years := children["a_0"].(map[string]any)
+		Expect(years).To(HaveKey("range"))
+		albums := years["aggs"].(map[string]any)["a_0"].(map[string]any)["terms"].(map[string]any)
+		Expect(albums["field"]).To(Equal("audio.album"))
+	})
+
+	It("reads nested results below their bucket, terms below a range below terms too", func() {
+		out := parse(`{"a_0": {"buckets": [
+			{"key": "Saxon", "doc_count": 2,
+				"a_0": {"buckets": [
+					{"key": "..1980", "doc_count": 2, "a_0": {"buckets": [{"key": "Wheels of Steel", "doc_count": 2}]}},
+					{"key": "1980..", "doc_count": 0, "a_0": {"buckets": []}}
+				]},
+				"a_1": {"count": 2, "min": 1971.0, "max": 1975.0, "sum": 3946.0}},
+			{"key": "Accept", "doc_count": 1,
+				"a_0": {"buckets": [
+					{"key": "..1980", "doc_count": 0, "a_0": {"buckets": []}},
+					{"key": "1980..", "doc_count": 1, "a_0": {"buckets": [{"key": "Breaker", "doc_count": 1}]}}
+				]},
+				"a_1": {"count": 1, "min": 1981.0, "max": 1981.0, "sum": 1981.0}}
+		]}}`, &searchsvc.AggregationOption{
+			Field: "audio.artist",
+			SubAggregations: []*searchsvc.AggregationOption{
+				{Field: "audio.year", BucketDefinition: &searchsvc.BucketDefinition{Ranges: []*searchsvc.BucketRange{{To: "1980"}, {From: "1980"}}},
+					SubAggregations: []*searchsvc.AggregationOption{{Field: "audio.album"}}},
+				{Field: "audio.year", MetricDefinition: &searchsvc.MetricDefinition{Kind: searchsvc.MetricKind_METRIC_KIND_SUM}},
+			},
+		})
+		Expect(keys(out[0])).To(Equal([]string{"Saxon=2", "Accept=1"}))
+
+		saxon := out[0].GetBuckets()[0].GetSubAggregations()
+		Expect(saxon).To(HaveLen(2))
+		Expect(keys(saxon[0])).To(Equal([]string{"..1980=2", "1980..=0"}))
+		Expect(keys(saxon[0].GetBuckets()[0].GetSubAggregations()[0])).To(Equal([]string{"Wheels of Steel=2"}), "only the albums of this artist")
+		Expect(saxon[0].GetBuckets()[1].GetSubAggregations()[0].GetBuckets()).To(BeEmpty())
+		Expect(saxon[1].GetMetric().GetSum()).To(Equal(3946.0))
+
+		accept := out[0].GetBuckets()[1].GetSubAggregations()
+		Expect(keys(accept[0].GetBuckets()[1].GetSubAggregations()[0])).To(Equal([]string{"Breaker=1"}))
+	})
 })
diff --git a/services/search/pkg/parity/README.md b/services/search/pkg/parity/README.md
index 8488374ee6..0272eb4ff2 100644
--- a/services/search/pkg/parity/README.md
+++ b/services/search/pkg/parity/README.md
@@ -745,11 +745,20 @@ Fixtures:
 | AGG-07 | `mediatype:image` reads `photo.takenDateTime buckets per date range, an empty range counts zero` | photo.take...00:00Z=0, photo.take...00:00Z=1, photo.take...00:00Z=2, photo.take...00:00Z=2 | photo.take...00:00Z=0, photo.take...00:00Z=1, photo.take...00:00Z=2, photo.take...00:00Z=2 | photo.take...00:00Z=0, photo.take...00:00Z=1, photo.take...00:00Z=2, photo.take...00:00Z=2 | ✅ |
 | AGG-08 | `mediatype:image` reads `open-ended date ranges` | photo.take...00:00Z=3, photo.take...:00Z..=1 | photo.take...00:00Z=3, photo.take...:00Z..=1 | photo.take...00:00Z=3, photo.take...:00Z..=1 | ✅ |
 | AGG-09 | `mediatype:image` reads `malformed date range bound` | bad request | bad request | bad request | ✅ |
+| AGG-10 | `mediatype:audio` reads `album buckets nested in artist buckets` | audio.arti... Steel=2, audio.arti...Bomber=2, audio.arti...Spades=1, audio.artist Motörhead=3, audio.artist Saxon=2 | audio.arti... Steel=2, audio.arti...Bomber=2, audio.arti...Spades=1, audio.artist Motörhead=3, audio.artist Saxon=2 | audio.arti... Steel=2, audio.arti...Bomber=2, audio.arti...Spades=1, audio.artist Motörhead=3, audio.artist Saxon=2 | ✅ |
+| AGG-11 | `mediatype:audio` reads `sum and avg of audio.year per artist` | audio.arti... count=2, audio.arti... count=3, audio.arti...sum=3946, audio.arti...sum=5982, audio.artist Motörhead=3, audio.artist Saxon=2 | audio.arti... count=2, audio.arti... count=3, audio.arti...sum=3946, audio.arti...sum=5982, audio.artist Motörhead=3, audio.artist Saxon=2 | audio.arti... count=2, audio.arti... count=3, audio.arti...sum=3946, audio.arti...sum=5982, audio.artist Motörhead=3, audio.artist Saxon=2 | ✅ |
+| AGG-12 | `mediatype:audio` reads `artist buckets nested in audio.year decades` | audio.year 1970..1980=2, audio.year 1980..1990=1, audio.year 1990..2000=1, audio.year 2000..2010=3, audio.year... Saxon=2, audio.year...örhead=1, audio.year...örhead=1, audio.year...örhead=1 | audio.year 1970..1980=2, audio.year 1980..1990=1, audio.year 1990..2000=1, audio.year 2000..2010=3, audio.year... Saxon=2, audio.year...örhead=1, audio.year...örhead=1, audio.year...örhead=1 | audio.year 1970..1980=2, audio.year 1980..1990=1, audio.year 1990..2000=1, audio.year 2000..2010=3, audio.year... Saxon=2, audio.year...örhead=1, audio.year...örhead=1, audio.year...örhead=1 | ✅ |
+| AGG-13 | `mediatype:audio` reads `max audio.year per album per artist, three levels` | audio.arti... Steel=2, audio.arti...Bomber=2, audio.arti...Spades=1, audio.arti...max=1975, audio.arti...max=1999, audio.arti...max=2001, audio.artist Motörhead=3, audio.artist Saxon=2 | audio.arti... Steel=2, audio.arti...Bomber=2, audio.arti...Spades=1, audio.arti...max=1975, audio.arti...max=1999, audio.arti...max=2001, audio.artist Motörhead=3, audio.artist Saxon=2 | audio.arti... Steel=2, audio.arti...Bomber=2, audio.arti...Spades=1, audio.arti...max=1975, audio.arti...max=1999, audio.arti...max=2001, audio.artist Motörhead=3, audio.artist Saxon=2 | ✅ |
+| AGG-14 | `mediatype:image` reads `MimeType buckets nested in open-ended date ranges` | photo.take...00:00Z=3, photo.take...:00Z..=1, photo.take...e/jpeg=1, photo.take...e/jpeg=3 | photo.take...00:00Z=3, photo.take...:00Z..=1, photo.take...e/jpeg=1, photo.take...e/jpeg=3 | error | ❌ known |
+| AGG-15 | `mediatype:audio` reads `nested aggregations cover every match on a page of one` | 1 of 7 matches, audio.arti... Steel=2, audio.arti...Bomber=2, audio.arti...Spades=1, audio.artist Motörhead=3, audio.artist Saxon=2, audio.year sum=13942 | 1 of 7 matches, audio.arti... Steel=2, audio.arti...Bomber=2, audio.arti...Spades=1, audio.artist Motörhead=3, audio.artist Saxon=2, audio.year sum=13942 | 1 of 7 matches, audio.arti... Steel=2, audio.arti...Bomber=2, audio.arti...Spades=1, audio.artist Motörhead=3, audio.artist Saxon=2, audio.year sum=13942 | ✅ |
 | AGG-18 | `mediatype:audio` reads `two range aggregations and a metric on audio.year stay apart` | audio.year ..1980=2, audio.year ..2000=4, audio.year 1980..=5, audio.year 2000..=3, audio.year max=2009 | audio.year ..1980=2, audio.year ..2000=4, audio.year 1980..=5, audio.year 2000..=3, audio.year max=2009 | audio.year ..1980=2, audio.year ..2000=4, audio.year 1980..=5, audio.year 2000..=3, audio.year max=2009 | ✅ |
+| AGG-20 | `mediatype:audio` reads `album buckets in audio.year ranges in artist buckets` | audio.arti... Steel=2, audio.arti.....1980=0, audio.arti.....1980=2, audio.arti...1980..=0, audio.arti...1980..=3, audio.arti...Bomber=2, audio.arti...Spades=1, audio.artist Motörhead=3, … +1 more | audio.arti... Steel=2, audio.arti.....1980=0, audio.arti.....1980=2, audio.arti...1980..=0, audio.arti...1980..=3, audio.arti...Bomber=2, audio.arti...Spades=1, audio.artist Motörhead=3, … +1 more | audio.arti... Steel=2, audio.arti.....1980=0, audio.arti.....1980=2, audio.arti...1980..=0, audio.arti...1980..=3, audio.arti...Bomber=2, audio.arti...Spades=1, audio.artist Motörhead=3, … +1 more | ✅ |
 | AGG-21 | `mediatype:image` reads `a metric without a single value has none` | audio.year avg none, audio.year min none, audio.year sum none | audio.year avg none, audio.year min none, audio.year sum none | audio.year avg none, audio.year min none, audio.year sum none | ✅ |
 | AGG-34 | `mediatype:audio` reads `term buckets on the numeric audio.year` | audio.year 1971=1, audio.year 1975=1, audio.year 1982=1, audio.year 1999=1, audio.year 2001=1, audio.year 2005=1, audio.year 2009=1 | audio.year 1971=1, audio.year 1975=1, audio.year 1982=1, audio.year 1999=1, audio.year 2001=1, audio.year 2005=1, audio.year 2009=1 | audio.year 1971=1, audio.year 1975=1, audio.year 1982=1, audio.year 1999=1, audio.year 2001=1, audio.year 2005=1, audio.year 2009=1 | ✅ |
+| AGG-35 | `mediatype:audio` reads `audio.year buckets nested in artist buckets` | audio.arti...r 1971=1, audio.arti...r 1975=1, audio.arti...r 1982=1, audio.arti...r 1999=1, audio.arti...r 2001=1, audio.artist Motörhead=3, audio.artist Saxon=2 | audio.arti...r 1971=1, audio.arti...r 1975=1, audio.arti...r 1982=1, audio.arti...r 1999=1, audio.arti...r 2001=1, audio.artist Motörhead=3, audio.artist Saxon=2 | audio.arti...r 1971=1, audio.arti...r 1975=1, audio.arti...r 1982=1, audio.arti...r 1999=1, audio.arti...r 2001=1, audio.artist Motörhead=3, audio.artist Saxon=2 | ✅ |
 | AGG-37 | `mediatype:audio` reads `term buckets on the bool audio.hasDrm, spelled true and false` | audio.hasDrm false=1, audio.hasDrm true=1 | audio.hasDrm false=1, audio.hasDrm true=1 | audio.hasDrm false=1, audio.hasDrm true=1 | ✅ |
 | AGG-39 | `mediatype:audio` reads `no bucket for the empty Title of every match` | no match | no match | no match | ✅ |
+| AGG-40 | `mediatype:audio` reads `no bucket for the empty Title, with sub-aggregations neither` | no match | no match | no match | ✅ |
 | AGG-41 | `mediatype:audio` reads `matches of the same score in the order of their ids, page after page` | 3 of 7 matches, a.mp3, b.mp3, c.mp3 | 3 of 7 matches, a.mp3, b.mp3, c.mp3 | 3 of 7 matches, a.mp3, b.mp3, c.mp3 | ✅ |
 
 ### cardinality
@@ -773,3 +782,4 @@ Fixtures:
 | AGG-17 | `mediatype:audio` reads `one bucket per artist, cardinality above the page size` | 10050 items | audio.arti...-00000=1, audio.arti...-00001=1, audio.arti...-00002=1, audio.arti...-00003=1, audio.arti...-00004=1, audio.arti...-00005=1, audio.arti...-00006=1, audio.arti...-00007=1, … +10042 more | audio.arti...-00000=1, audio.arti...-00001=1, audio.arti...-00002=1, audio.arti...-00003=1, audio.arti...-00004=1, audio.arti...-00005=1, audio.arti...-00006=1, audio.arti...-00007=1, … +10042 more | ✅ |
 | AGG-42 | `mediatype:audio` reads `the total counts every match` | 1 of 10050 matches | 1 of 10050 matches | 1 of 10050 matches | ✅ |
 | AGG-43 | `mediatype:audio` reads `a page reaching beyond the first 10000 matches is refused` | bad request | bad request | bad request | ✅ |
+| AGG-44 | `mediatype:audio` reads `more than 65535 buckets in one request are refused` | bad request | bad request | bad request | ✅ |
diff --git a/services/search/pkg/parity/aggregations_test.go b/services/search/pkg/parity/aggregations_test.go
index 46ad50148e..1bb94fdab0 100644
--- a/services/search/pkg/parity/aggregations_test.go
+++ b/services/search/pkg/parity/aggregations_test.go
@@ -159,6 +159,70 @@ func aggregationCases() []aggCase {
 				&searchService.BucketRange{From: "2018-08-11T00:00:00Z", To: "not-a-date"},
 			)}},
 			wantBadRequest: true},
+		{id: 10, query: "mediatype:audio", reads: "album buckets nested in artist buckets",
+			aggs: []*searchService.AggregationOption{{Field: "audio.artist", SubAggregations: []*searchService.AggregationOption{{Field: "audio.album"}}}},
+			want: []string{
+				"audio.artist Saxon=2", "audio.artist Saxon=2 / audio.album Wheels of Steel=2",
+				"audio.artist Motörhead=3", "audio.artist Motörhead=3 / audio.album Bomber=2", "audio.artist Motörhead=3 / audio.album Ace of Spades=1",
+			}},
+		{id: 11, query: "mediatype:audio", reads: "sum and avg of audio.year per artist",
+			aggs: []*searchService.AggregationOption{{Field: "audio.artist", SubAggregations: []*searchService.AggregationOption{
+				metric("audio.year", searchService.MetricKind_METRIC_KIND_SUM),
+				metric("audio.year", searchService.MetricKind_METRIC_KIND_AVG),
+			}}},
+			want: []string{
+				"audio.artist Saxon=2", "audio.artist Saxon=2 / audio.year sum=3946", "audio.artist Saxon=2 / audio.year avg sum=3946 count=2",
+				"audio.artist Motörhead=3", "audio.artist Motörhead=3 / audio.year sum=5982", "audio.artist Motörhead=3 / audio.year avg sum=5982 count=3",
+			}},
+		{id: 12, query: "mediatype:audio", reads: "artist buckets nested in audio.year decades",
+			aggs: []*searchService.AggregationOption{{Field: "audio.year", BucketDefinition: ranges(
+				&searchService.BucketRange{From: "1970", To: "1980"},
+				&searchService.BucketRange{From: "1980", To: "1990"},
+				&searchService.BucketRange{From: "1990", To: "2000"},
+				&searchService.BucketRange{From: "2000", To: "2010"},
+			), SubAggregations: []*searchService.AggregationOption{{Field: "audio.artist"}}}},
+			want: []string{
+				"audio.year 1970..1980=2", "audio.year 1970..1980=2 / audio.artist Saxon=2",
+				"audio.year 1980..1990=1", "audio.year 1980..1990=1 / audio.artist Motörhead=1",
+				"audio.year 1990..2000=1", "audio.year 1990..2000=1 / audio.artist Motörhead=1",
+				"audio.year 2000..2010=3", "audio.year 2000..2010=3 / audio.artist Motörhead=1",
+			}},
+		{id: 13, query: "mediatype:audio", reads: "max audio.year per album per artist, three levels",
+			aggs: []*searchService.AggregationOption{{Field: "audio.artist", SubAggregations: []*searchService.AggregationOption{
+				{Field: "audio.album", SubAggregations: []*searchService.AggregationOption{
+					metric("audio.year", searchService.MetricKind_METRIC_KIND_MAX),
+				}},
+			}}},
+			want: []string{
+				"audio.artist Saxon=2", "audio.artist Saxon=2 / audio.album Wheels of Steel=2", "audio.artist Saxon=2 / audio.album Wheels of Steel=2 / audio.year max=1975",
+				"audio.artist Motörhead=3", "audio.artist Motörhead=3 / audio.album Bomber=2", "audio.artist Motörhead=3 / audio.album Bomber=2 / audio.year max=1999",
+				"audio.artist Motörhead=3 / audio.album Ace of Spades=1", "audio.artist Motörhead=3 / audio.album Ace of Spades=1 / audio.year max=2001",
+			}},
+		{id: 14, query: "mediatype:image", reads: "MimeType buckets nested in open-ended date ranges",
+			aggs: []*searchService.AggregationOption{{Field: "photo.takenDateTime", BucketDefinition: ranges(
+				&searchService.BucketRange{To: "2019-01-01T00:00:00Z"},
+				&searchService.BucketRange{From: "2019-01-01T00:00:00Z"},
+			), SubAggregations: []*searchService.AggregationOption{{Field: "MimeType"}}}},
+			want: []string{
+				"photo.takenDateTime ..2019-01-01T00:00:00Z=3", "photo.takenDateTime ..2019-01-01T00:00:00Z=3 / MimeType image/jpeg=3",
+				"photo.takenDateTime 2019-01-01T00:00:00Z..=1", "photo.takenDateTime 2019-01-01T00:00:00Z..=1 / MimeType image/jpeg=1",
+			},
+			// OpenSearch maps MimeType as wildcard, which serves no doc values to
+			// aggregate on; the follow-up is an aggregatable keyword sibling
+			engineOverrides: map[string]override{"opensearch": {want: []string{"error"}}}},
+		{id: 15, query: "mediatype:audio", reads: "nested aggregations cover every match on a page of one",
+			pageSize: 1,
+			aggs: []*searchService.AggregationOption{
+				{Field: "audio.artist", SubAggregations: []*searchService.AggregationOption{{Field: "audio.album"}}},
+				metric("audio.year", searchService.MetricKind_METRIC_KIND_SUM),
+			},
+			want: []string{
+				"1 of 7 matches",
+				"audio.artist Saxon=2", "audio.artist Saxon=2 / audio.album Wheels of Steel=2",
+				"audio.artist Motörhead=3", "audio.artist Motörhead=3 / audio.album Bomber=2", "audio.artist Motörhead=3 / audio.album Ace of Spades=1",
+				"audio.year sum=13942",
+			}},
+		// 16 and 19 pinned malformed bounds twice over, 17 is the cardinality case below
 		{id: 18, query: "mediatype:audio", reads: "two range aggregations and a metric on audio.year stay apart",
 			aggs: []*searchService.AggregationOption{
 				{Field: "audio.year", BucketDefinition: ranges(&searchService.BucketRange{To: "1980"}, &searchService.BucketRange{From: "1980"})},
@@ -170,6 +234,21 @@ func aggregationCases() []aggCase {
 				"audio.year ..2000=4", "audio.year 2000..=3",
 				"audio.year max=2009",
 			}},
+		{id: 20, query: "mediatype:audio", reads: "album buckets in audio.year ranges in artist buckets",
+			aggs: []*searchService.AggregationOption{{Field: "audio.artist", SubAggregations: []*searchService.AggregationOption{{
+				Field:            "audio.year",
+				BucketDefinition: ranges(&searchService.BucketRange{To: "1980"}, &searchService.BucketRange{From: "1980"}),
+				SubAggregations:  []*searchService.AggregationOption{{Field: "audio.album"}},
+			}}}},
+			want: []string{
+				"audio.artist Saxon=2",
+				"audio.artist Saxon=2 / audio.year ..1980=2", "audio.artist Saxon=2 / audio.year ..1980=2 / audio.album Wheels of Steel=2",
+				"audio.artist Saxon=2 / audio.year 1980..=0",
+				"audio.artist Motörhead=3",
+				"audio.artist Motörhead=3 / audio.year ..1980=0",
+				"audio.artist Motörhead=3 / audio.year 1980..=3",
+				"audio.artist Motörhead=3 / audio.year 1980..=3 / audio.album Bomber=2", "audio.artist Motörhead=3 / audio.year 1980..=3 / audio.album Ace of Spades=1",
+			}},
 		{id: 21, query: "mediatype:image", reads: "a metric without a single value has none",
 			aggs: []*searchService.AggregationOption{
 				metric("audio.year", searchService.MetricKind_METRIC_KIND_SUM),
@@ -183,11 +262,20 @@ func aggregationCases() []aggCase {
 				"audio.year 1971=1", "audio.year 1975=1", "audio.year 1982=1", "audio.year 1999=1",
 				"audio.year 2001=1", "audio.year 2005=1", "audio.year 2009=1",
 			}},
+		{id: 35, query: "mediatype:audio", reads: "audio.year buckets nested in artist buckets",
+			aggs: []*searchService.AggregationOption{{Field: "audio.artist", SubAggregations: []*searchService.AggregationOption{{Field: "audio.year"}}}},
+			want: []string{
+				"audio.artist Saxon=2", "audio.artist Saxon=2 / audio.year 1971=1", "audio.artist Saxon=2 / audio.year 1975=1",
+				"audio.artist Motörhead=3", "audio.artist Motörhead=3 / audio.year 1982=1",
+				"audio.artist Motörhead=3 / audio.year 1999=1", "audio.artist Motörhead=3 / audio.year 2001=1",
+			}},
 		{id: 37, query: "mediatype:audio", reads: "term buckets on the bool audio.hasDrm, spelled true and false",
 			aggs: []*searchService.AggregationOption{{Field: "audio.hasDrm"}},
 			want: []string{"audio.hasDrm true=1", "audio.hasDrm false=1"}},
 		{id: 39, query: "mediatype:audio", reads: "no bucket for the empty Title of every match",
 			aggs: []*searchService.AggregationOption{{Field: "Title"}}},
+		{id: 40, query: "mediatype:audio", reads: "no bucket for the empty Title, with sub-aggregations neither",
+			aggs: []*searchService.AggregationOption{{Field: "Title", SubAggregations: []*searchService.AggregationOption{{Field: "audio.artist"}}}}},
 		{id: 41, query: "mediatype:audio", reads: "matches of the same score in the order of their ids, page after page",
 			pageSize: 3, listMatches: true,
 			want: []string{"3 of 7 matches", "a.mp3", "b.mp3", "c.mp3"}},
@@ -195,7 +283,7 @@ func aggregationCases() []aggCase {
 }
 
 // renderAggregations flattens an answer into comparable strings, one per
-// bucket or metric.
+// bucket or metric. A nested result renders under its bucket, joined by " / ".
 func renderAggregations(resp *searchService.SearchIndexResponse, err error) []string {
 	if err != nil {
 		if _, ok := err.(errtypes.BadRequest); ok {
@@ -227,7 +315,11 @@ func renderAggregation(prefix string, a *searchService.AggregationResult) []stri
 
 	out := []string{}
 	for _, b := range a.Buckets {
-		out = append(out, prefix+fmt.Sprintf("%s %s=%d", a.Field, b.Key, b.Count))
+		line := prefix + fmt.Sprintf("%s %s=%d", a.Field, b.Key, b.Count)
+		out = append(out, line)
+		for _, sub := range b.SubAggregations {
+			out = append(out, renderAggregation(line+" / ", sub)...)
+		}
 	}
 
 	return out
@@ -243,6 +335,15 @@ func cardinalityFixtures() []search.Resource {
 	return docs
 }
 
+// nested puts a terms aggregation on each field into the one before it.
+func nested(fields ...string) *searchService.AggregationOption {
+	opt := &searchService.AggregationOption{Field: fields[0]}
+	if len(fields) > 1 {
+		opt.SubAggregations = []*searchService.AggregationOption{nested(fields[1:]...)}
+	}
+	return opt
+}
+
 func cardinalityCases() []aggCase {
 	count := 10050
 	return []aggCase{
@@ -255,6 +356,9 @@ func cardinalityCases() []aggCase {
 		{id: 43, query: "mediatype:audio", reads: "a page reaching beyond the first 10000 matches is refused",
 			pageSize:       10001,
 			wantBadRequest: true},
+		{id: 44, query: "mediatype:audio", reads: "more than 65535 buckets in one request are refused",
+			aggs:           []*searchService.AggregationOption{nested("audio.artist", "Name", "ID", "RootID", "ParentID", "audio.album", "audio.year")},
+			wantBadRequest: true},
 	}
 }