results cache: add new stat `query_length_served` to measure cache effectiveness (#11589)

**What this PR does / why we need it**:
cache hit rate that is currently being measured using metrics and stats
does not account for the fact that a cache hit could return partial
results. When we query the cache for a key, we get back a list of
extents and these need no cover the entire (split) range of the cache
key.

This pr adds a new cache stat called `query_length_served` to better
measure the cache effectiveness.
```
query_length - sum(length of downstream queries)
```

**Which issue(s) this PR fixes**:
Fixes #<issue number>

**Special notes for your reviewer**:

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] Documentation added
- [x] Tests updated
- [x] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](d10549e3ec)
- [ ] If the change is deprecating or removing a configuration option,
update the `deprecated-config.yaml` and `deleted-config.yaml` files
respectively in the `tools/deprecated-config-checker` directory.
[Example
PR](0d4416a4b0)

CHANGELOG.md

@@ -45,6 +45,7 @@
 * [11284](https://github.com/grafana/loki/pull/11284) **ashwanthgoli** Config: Adds `frontend.max-query-capacity` to tune per-tenant query capacity.
 * [11539](https://github.com/grafana/loki/pull/11539) **kaviraj,ashwanthgoli** Support caching /series and /labels query results
 * [11545](https://github.com/grafana/loki/pull/11545) **dannykopping** Force correct memcached timeout when fetching chunks.
+* [11589](https://github.com/grafana/loki/pull/11589) **ashwanthgoli** Results Cache: Adds `query_length_served` cache stat to measure the length of the query served from cache.
 
 ##### Fixes
 * [11074](https://github.com/grafana/loki/pull/11074) **hainenber** Fix panic in lambda-promtail due to mishandling of empty DROP_LABELS env var.

pkg/logql/metrics.go

@@ -163,6 +163,7 @@ func RecordRangeAndInstantQueryMetrics(
 		"cache_result_req", stats.Caches.Result.EntriesRequested,
 		"cache_result_hit", stats.Caches.Result.EntriesFound,
 		"cache_result_download_time", stats.Caches.Result.CacheDownloadTime(),
+		"cache_result_query_length_served", stats.Caches.Result.CacheQueryLengthServed(),
 	}...)
 
 	logValues = append(logValues, tagsToKeyValues(queryTags)...)
@@ -228,6 +229,7 @@ func RecordLabelQueryMetrics(
 		"cache_label_results_hit", stats.Caches.LabelResult.EntriesFound,
 		"cache_label_results_stored", stats.Caches.LabelResult.EntriesStored,
 		"cache_label_results_download_time", stats.Caches.LabelResult.CacheDownloadTime(),
+		"cache_label_results_query_length_served", stats.Caches.LabelResult.CacheQueryLengthServed(),
 	)
 
 	execLatency.WithLabelValues(status, queryType, "").Observe(stats.Summary.ExecTime)
@@ -341,6 +343,7 @@ func RecordSeriesQueryMetrics(ctx context.Context, log log.Logger, start, end ti
 		"cache_series_results_hit", stats.Caches.SeriesResult.EntriesFound,
 		"cache_series_results_stored", stats.Caches.SeriesResult.EntriesStored,
 		"cache_series_results_download_time", stats.Caches.SeriesResult.CacheDownloadTime(),
+		"cache_series_results_query_length_served", stats.Caches.SeriesResult.CacheQueryLengthServed(),
 	)
 
 	if shard != nil {
@@ -428,6 +431,7 @@ func RecordVolumeQueryMetrics(ctx context.Context, log log.Logger, start, end ti
 		"cache_volume_results_hit", stats.Caches.VolumeResult.EntriesFound,
 		"cache_volume_results_stored", stats.Caches.VolumeResult.EntriesStored,
 		"cache_volume_results_download_time", stats.Caches.VolumeResult.CacheDownloadTime(),
+		"cache_volume_results_query_length_served", stats.Caches.VolumeResult.CacheQueryLengthServed(),
 	)
 
 	execLatency.WithLabelValues(status, queryType, "").Observe(stats.Summary.ExecTime)

pkg/logql/metrics_test.go

@@ -108,16 +108,17 @@ func TestLogLabelsQuery(t *testing.T) {
 		},
 		Caches: stats.Caches{
 			LabelResult: stats.Cache{
-				EntriesRequested: 2,
-				EntriesFound:     1,
-				EntriesStored:    1,
-				DownloadTime:     80,
+				EntriesRequested:  2,
+				EntriesFound:      1,
+				EntriesStored:     1,
+				DownloadTime:      80,
+				QueryLengthServed: 10,
 			},
 		},
 	})
 	require.Regexp(t,
 		fmt.Sprintf(
-			"level=info org_id=foo traceID=%s sampled=true latency=slow query_type=labels splits=0 start=.* end=.* start_delta=1h0m0.* end_delta=.* length=1h0m0s duration=25.25s status=200 label=foo query= query_hash=2166136261 total_entries=12 cache_label_results_req=2 cache_label_results_hit=1 cache_label_results_stored=1 cache_label_results_download_time=80ns\n",
+			"level=info org_id=foo traceID=%s sampled=true latency=slow query_type=labels splits=0 start=.* end=.* start_delta=1h0m0.* end_delta=.* length=1h0m0s duration=25.25s status=200 label=foo query= query_hash=2166136261 total_entries=12 cache_label_results_req=2 cache_label_results_hit=1 cache_label_results_stored=1 cache_label_results_download_time=80ns cache_label_results_query_length_served=10ns\n",
 			sp.Context().(jaeger.SpanContext).SpanID().String(),
 		),
 		buf.String())
@@ -142,16 +143,17 @@ func TestLogSeriesQuery(t *testing.T) {
 		},
 		Caches: stats.Caches{
 			SeriesResult: stats.Cache{
-				EntriesRequested: 2,
-				EntriesFound:     1,
-				EntriesStored:    1,
-				DownloadTime:     80,
+				EntriesRequested:  2,
+				EntriesFound:      1,
+				EntriesStored:     1,
+				DownloadTime:      80,
+				QueryLengthServed: 10,
 			},
 		},
 	})
 	require.Regexp(t,
 		fmt.Sprintf(
-			"level=info org_id=foo traceID=%s sampled=true latency=slow query_type=series splits=0 start=.* end=.* start_delta=1h0m0.* end_delta=.* length=1h0m0s duration=25.25s status=200 match=\"{container_name=.*\"}:{app=.*}\" query_hash=23523089 total_entries=10 cache_series_results_req=2 cache_series_results_hit=1 cache_series_results_stored=1 cache_series_results_download_time=80ns\n",
+			"level=info org_id=foo traceID=%s sampled=true latency=slow query_type=series splits=0 start=.* end=.* start_delta=1h0m0.* end_delta=.* length=1h0m0s duration=25.25s status=200 match=\"{container_name=.*\"}:{app=.*}\" query_hash=23523089 total_entries=10 cache_series_results_req=2 cache_series_results_hit=1 cache_series_results_stored=1 cache_series_results_download_time=80ns cache_series_results_query_length_served=10ns\n",
 			sp.Context().(jaeger.SpanContext).SpanID().String(),
 		),
 		buf.String())

pkg/logqlmodel/stats/context.go

@@ -231,12 +231,17 @@ func (c *Cache) Merge(m Cache) {
 	c.BytesSent += m.BytesSent
 	c.BytesReceived += m.BytesReceived
 	c.DownloadTime += m.DownloadTime
+	c.QueryLengthServed += m.QueryLengthServed
 }
 
 func (c *Cache) CacheDownloadTime() time.Duration {
 	return time.Duration(c.DownloadTime)
 }
 
+func (c *Cache) CacheQueryLengthServed() time.Duration {
+	return time.Duration(c.QueryLengthServed)
+}
+
 func (r *Result) MergeSplit(m Result) {
 	m.Summary.Splits = 1
 	r.Merge(m)
@@ -429,6 +434,16 @@ func (c *Context) AddCacheRequest(t CacheType, i int) {
 	atomic.AddInt32(&stats.Requests, int32(i))
 }
 
+// AddCacheQueryLengthServed measures the length of the query served from cache
+func (c *Context) AddCacheQueryLengthServed(t CacheType, i time.Duration) {
+	stats := c.getCacheStatsByType(t)
+	if stats == nil {
+		return
+	}
+
+	atomic.AddInt64(&stats.QueryLengthServed, int64(i))
+}
+
 func (c *Context) AddSplitQueries(num int64) {
 	atomic.AddInt64(&c.result.Summary.Splits, num)
 }

pkg/logqlmodel/stats/context_test.go

@@ -210,7 +210,8 @@ func TestResult_Merge(t *testing.T) {
 				EntriesFound:     2,
 			},
 			Result: Cache{
-				EntriesStored: 3,
+				EntriesStored:     3,
+				QueryLengthServed: int64(3 * time.Hour),
 			},
 		},
 		Summary: Summary{
@@ -272,7 +273,8 @@ func TestResult_Merge(t *testing.T) {
 				EntriesFound:     2 * 2,
 			},
 			Result: Cache{
-				EntriesStored: 2 * 3,
+				EntriesStored:     2 * 3,
+				QueryLengthServed: int64(2 * 3 * time.Hour),
 			},
 		},
 		Summary: Summary{
@@ -325,6 +327,7 @@ func TestCaches(t *testing.T) {
 
 	statsCtx.AddCacheRequest(ChunkCache, 5)
 	statsCtx.AddCacheEntriesStored(ResultCache, 3)
+	statsCtx.AddCacheQueryLengthServed(ResultCache, 3*time.Hour)
 	statsCtx.AddCacheEntriesRequested(IndexCache, 22)
 	statsCtx.AddCacheBytesRetrieved(ChunkCache, 1024)
 	statsCtx.AddCacheBytesSent(ChunkCache, 512)
@@ -341,7 +344,8 @@ func TestCaches(t *testing.T) {
 			EntriesFound:     2,
 		},
 		Result: Cache{
-			EntriesStored: 3,
+			EntriesStored:     3,
+			QueryLengthServed: int64(time.Hour * 3),
 		},
 	}, statsCtx.Caches())
 }

pkg/logqlmodel/stats/stats.pb.go

@@ -661,13 +661,14 @@ func (m *Chunk) GetDecompressedStructuredMetadataBytes() int64 {
 }
 
 type Cache struct {
-	EntriesFound     int32 `protobuf:"varint,1,opt,name=entriesFound,proto3" json:"entriesFound"`
-	EntriesRequested int32 `protobuf:"varint,2,opt,name=entriesRequested,proto3" json:"entriesRequested"`
-	EntriesStored    int32 `protobuf:"varint,3,opt,name=entriesStored,proto3" json:"entriesStored"`
-	BytesReceived    int64 `protobuf:"varint,4,opt,name=bytesReceived,proto3" json:"bytesReceived"`
-	BytesSent        int64 `protobuf:"varint,5,opt,name=bytesSent,proto3" json:"bytesSent"`
-	Requests         int32 `protobuf:"varint,6,opt,name=requests,proto3" json:"requests"`
-	DownloadTime     int64 `protobuf:"varint,7,opt,name=downloadTime,proto3" json:"downloadTime"`
+	EntriesFound      int32 `protobuf:"varint,1,opt,name=entriesFound,proto3" json:"entriesFound"`
+	EntriesRequested  int32 `protobuf:"varint,2,opt,name=entriesRequested,proto3" json:"entriesRequested"`
+	EntriesStored     int32 `protobuf:"varint,3,opt,name=entriesStored,proto3" json:"entriesStored"`
+	BytesReceived     int64 `protobuf:"varint,4,opt,name=bytesReceived,proto3" json:"bytesReceived"`
+	BytesSent         int64 `protobuf:"varint,5,opt,name=bytesSent,proto3" json:"bytesSent"`
+	Requests          int32 `protobuf:"varint,6,opt,name=requests,proto3" json:"requests"`
+	DownloadTime      int64 `protobuf:"varint,7,opt,name=downloadTime,proto3" json:"downloadTime"`
+	QueryLengthServed int64 `protobuf:"varint,8,opt,name=queryLengthServed,proto3" json:"queryLengthServed"`
 }
 
 func (m *Cache) Reset()      { *m = Cache{} }
@@ -751,6 +752,13 @@ func (m *Cache) GetDownloadTime() int64 {
 	return 0
 }
 
+func (m *Cache) GetQueryLengthServed() int64 {
+	if m != nil {
+		return m.QueryLengthServed
+	}
+	return 0
+}
+
 func init() {
 	proto.RegisterType((*Result)(nil), "stats.Result")
 	proto.RegisterType((*Caches)(nil), "stats.Caches")
@@ -765,82 +773,83 @@ func init() {
 func init() { proto.RegisterFile("pkg/logqlmodel/stats/stats.proto", fileDescriptor_6cdfe5d2aea33ebb) }
 
 var fileDescriptor_6cdfe5d2aea33ebb = []byte{
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+	0xf1, 0x7f, 0x01, 0x00, 0x00, 0xff, 0xff, 0xa8, 0xe8, 0xef, 0xe7, 0x47, 0x0e, 0x00, 0x00,
 }
 
 func (this *Result) Equal(that interface{}) bool {
@@ -1162,6 +1171,9 @@ func (this *Cache) Equal(that interface{}) bool {
 	if this.DownloadTime != that1.DownloadTime {
 		return false
 	}
+	if this.QueryLengthServed != that1.QueryLengthServed {
+		return false
+	}
 	return true
 }
 func (this *Result) GoString() string {
@@ -1275,7 +1287,7 @@ func (this *Cache) GoString() string {
 	if this == nil {
 		return "nil"
 	}
-	s := make([]string, 0, 11)
+	s := make([]string, 0, 12)
 	s = append(s, "&stats.Cache{")
 	s = append(s, "EntriesFound: "+fmt.Sprintf("%#v", this.EntriesFound)+",\n")
 	s = append(s, "EntriesRequested: "+fmt.Sprintf("%#v", this.EntriesRequested)+",\n")
@@ -1284,6 +1296,7 @@ func (this *Cache) GoString() string {
 	s = append(s, "BytesSent: "+fmt.Sprintf("%#v", this.BytesSent)+",\n")
 	s = append(s, "Requests: "+fmt.Sprintf("%#v", this.Requests)+",\n")
 	s = append(s, "DownloadTime: "+fmt.Sprintf("%#v", this.DownloadTime)+",\n")
+	s = append(s, "QueryLengthServed: "+fmt.Sprintf("%#v", this.QueryLengthServed)+",\n")
 	s = append(s, "}")
 	return strings.Join(s, "")
 }
@@ -1773,6 +1786,11 @@ func (m *Cache) MarshalToSizedBuffer(dAtA []byte) (int, error) {
 	_ = i
 	var l int
 	_ = l
+	if m.QueryLengthServed != 0 {
+		i = encodeVarintStats(dAtA, i, uint64(m.QueryLengthServed))
+		i--
+		dAtA[i] = 0x40
+	}
 	if m.DownloadTime != 0 {
 		i = encodeVarintStats(dAtA, i, uint64(m.DownloadTime))
 		i--
@@ -2030,6 +2048,9 @@ func (m *Cache) Size() (n int) {
 	if m.DownloadTime != 0 {
 		n += 1 + sovStats(uint64(m.DownloadTime))
 	}
+	if m.QueryLengthServed != 0 {
+		n += 1 + sovStats(uint64(m.QueryLengthServed))
+	}
 	return n
 }
 
@@ -2158,6 +2179,7 @@ func (this *Cache) String() string {
 		`BytesSent:` + fmt.Sprintf("%v", this.BytesSent) + `,`,
 		`Requests:` + fmt.Sprintf("%v", this.Requests) + `,`,
 		`DownloadTime:` + fmt.Sprintf("%v", this.DownloadTime) + `,`,
+		`QueryLengthServed:` + fmt.Sprintf("%v", this.QueryLengthServed) + `,`,
 		`}`,
 	}, "")
 	return s
@@ -3720,6 +3742,25 @@ func (m *Cache) Unmarshal(dAtA []byte) error {
 					break
 				}
 			}
+		case 8:
+			if wireType != 0 {
+				return fmt.Errorf("proto: wrong wireType = %d for field QueryLengthServed", wireType)
+			}
+			m.QueryLengthServed = 0
+			for shift := uint(0); ; shift += 7 {
+				if shift >= 64 {
+					return ErrIntOverflowStats
+				}
+				if iNdEx >= l {
+					return io.ErrUnexpectedEOF
+				}
+				b := dAtA[iNdEx]
+				iNdEx++
+				m.QueryLengthServed |= int64(b&0x7F) << shift
+				if b < 0x80 {
+					break
+				}
+			}
 		default:
 			iNdEx = preIndex
 			skippy, err := skipStats(dAtA[iNdEx:])

pkg/logqlmodel/stats/stats.proto

@@ -163,4 +163,5 @@ message Cache {
   int64 bytesSent = 5 [(gogoproto.jsontag) = "bytesSent"];
   int32 requests = 6 [(gogoproto.jsontag) = "requests"];
   int64 downloadTime = 7 [(gogoproto.jsontag) = "downloadTime"];
+  int64 queryLengthServed = 8 [(gogoproto.jsontag) = "queryLengthServed"];
 }

pkg/querier/queryrange/codec_test.go

@@ -1592,7 +1592,8 @@ var (
 				"bytesReceived": 0,
 				"bytesSent": 0,
 				"requests": 0,
-				"downloadTime": 0
+				"downloadTime": 0,
+				"queryLengthServed": 0
 			},
 			"index": {
 				"entriesFound": 0,
@@ -1601,7 +1602,8 @@ var (
 				"bytesReceived": 0,
 				"bytesSent": 0,
 				"requests": 0,
-				"downloadTime": 0
+				"downloadTime": 0,
+				"queryLengthServed": 0
 			},
 		  "statsResult": {
 				"entriesFound": 0,
@@ -1610,7 +1612,8 @@ var (
 				"bytesReceived": 0,
 				"bytesSent": 0,
 				"requests": 0,
-				"downloadTime": 0
+				"downloadTime": 0,
+				"queryLengthServed": 0
 			},
 		  "seriesResult": {
 				"entriesFound": 0,
@@ -1619,7 +1622,8 @@ var (
 				"bytesReceived": 0,
 				"bytesSent": 0,
 				"requests": 0,
-				"downloadTime": 0
+				"downloadTime": 0,
+				"queryLengthServed": 0
 			},
 		  "labelResult": {
 				"entriesFound": 0,
@@ -1628,7 +1632,8 @@ var (
 				"bytesReceived": 0,
 				"bytesSent": 0,
 				"requests": 0,
-				"downloadTime": 0
+				"downloadTime": 0,
+				"queryLengthServed": 0
 			},
 		  "volumeResult": {
 				"entriesFound": 0,
@@ -1637,7 +1642,8 @@ var (
 				"bytesReceived": 0,
 				"bytesSent": 0,
 				"requests": 0,
-				"downloadTime": 0
+				"downloadTime": 0,
+				"queryLengthServed": 0
 			},
 			"result": {
 				"entriesFound": 0,
@@ -1646,7 +1652,8 @@ var (
 				"bytesReceived": 0,
 				"bytesSent": 0,
 				"requests": 0,
-				"downloadTime": 0
+				"downloadTime": 0,
+				"queryLengthServed": 0
 			}
 		},
 		"summary": {

pkg/querier/queryrange/prometheus_test.go

@@ -65,7 +65,8 @@ var emptyStats = `"stats": {
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"index": {
 			"entriesFound": 0,
@@ -74,7 +75,8 @@ var emptyStats = `"stats": {
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"statsResult": {
 			"entriesFound": 0,
@@ -83,7 +85,8 @@ var emptyStats = `"stats": {
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"seriesResult": {
 			"entriesFound": 0,
@@ -92,7 +95,8 @@ var emptyStats = `"stats": {
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"labelResult": {
 			"entriesFound": 0,
@@ -101,7 +105,8 @@ var emptyStats = `"stats": {
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"volumeResult": {
 			"entriesFound": 0,
@@ -110,7 +115,8 @@ var emptyStats = `"stats": {
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"result": {
 			"entriesFound": 0,
@@ -119,7 +125,8 @@ var emptyStats = `"stats": {
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		}
 	},
 	"summary": {

pkg/storage/chunk/cache/resultscache/cache.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/grafana/dskit/tenant"
 
+	"github.com/grafana/loki/pkg/logqlmodel/stats"
 	"github.com/grafana/loki/pkg/storage/chunk/cache"
 	"github.com/grafana/loki/pkg/util/math"
 	"github.com/grafana/loki/pkg/util/spanlogger"
@@ -187,7 +188,11 @@ func (s ResultsCache) handleHit(ctx context.Context, r Request, extents []Extent
 	if err != nil {
 		return nil, nil, err
 	}
+
+	queryLenFromCache := r.GetEnd().Sub(r.GetStart())
+	st := stats.FromContext(ctx)
 	if len(requests) == 0 {
+		st.AddCacheQueryLengthServed(s.cache.GetCacheType(), queryLenFromCache)
 		response, err := s.merger.MergeResponse(responses...)
 		// No downstream requests so no need to write back to the cache.
 		return response, nil, err
@@ -204,6 +209,7 @@ func (s ResultsCache) handleHit(ctx context.Context, r Request, extents []Extent
 	}
 
 	for _, reqResp := range reqResps {
+		queryLenFromCache -= reqResp.Request.GetEnd().Sub(reqResp.Request.GetStart())
 		responses = append(responses, reqResp.Response)
 		if s.shouldCacheRes != nil && !s.shouldCacheRes(ctx, r, reqResp.Response, maxCacheTime) {
 			continue
@@ -267,6 +273,7 @@ func (s ResultsCache) handleHit(ctx context.Context, r Request, extents []Extent
 		return nil, nil, err
 	}
 
+	st.AddCacheQueryLengthServed(s.cache.GetCacheType(), queryLenFromCache)
 	response, err := s.merger.MergeResponse(responses...)
 	return response, mergedExtents, err
 }

pkg/storage/chunk/cache/resultscache/cache_test.go

@@ -370,6 +370,7 @@ func TestHandleHit(t *testing.T) {
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			sut := ResultsCache{
+				cache:             cache.NewMockCache(),
 				extractor:         MockExtractor{},
 				minCacheExtent:    10,
 				limits:            mockLimits{},
@@ -391,6 +392,62 @@ func TestHandleHit(t *testing.T) {
 	}
 }
 
+func TestHandleHit_queryLengthServed(t *testing.T) {
+	rc := ResultsCache{
+		cache:             &mockResultsCache{},
+		extractor:         MockExtractor{},
+		limits:            mockLimits{},
+		merger:            MockMerger{},
+		parallelismForReq: func(_ context.Context, tenantIDs []string, r Request) int { return 1 },
+		next: HandlerFunc(func(_ context.Context, req Request) (Response, error) {
+			return mkAPIResponse(req.GetStart().UnixMilli(), req.GetEnd().UnixMilli(), req.GetStep()), nil
+		}),
+	}
+
+	statsCtx, ctx := stats.NewContext(context.Background())
+	ctx = user.InjectOrgID(ctx, "1")
+
+	input := &MockRequest{
+		Start: time.UnixMilli(100),
+		End:   time.UnixMilli(130),
+		Step:  5,
+	}
+
+	// no cached response
+	_, _, err := rc.handleHit(ctx, input, nil, 0)
+	require.NoError(t, err)
+	require.Equal(t, int64(0), statsCtx.Caches().Result.QueryLengthServed)
+
+	// partial hit
+	extents := []Extent{
+		mkExtentWithStep(60, 80, 5),
+		mkExtentWithStep(90, 105, 5),
+	}
+	statsCtx, ctx = stats.NewContext(ctx)
+	_, _, err = rc.handleHit(ctx, input, extents, 0)
+	require.NoError(t, err)
+	require.Equal(t, int64(5*time.Millisecond), statsCtx.Caches().Result.QueryLengthServed)
+
+	extents = []Extent{
+		mkExtentWithStep(90, 105, 5),
+		mkExtentWithStep(110, 120, 5),
+	}
+	statsCtx, ctx = stats.NewContext(ctx)
+	_, _, err = rc.handleHit(ctx, input, extents, 0)
+	require.NoError(t, err)
+	require.Equal(t, int64(15*time.Millisecond), statsCtx.Caches().Result.QueryLengthServed)
+
+	// entire query served from cache
+	extents = []Extent{
+		mkExtentWithStep(90, 110, 5),
+		mkExtentWithStep(110, 130, 5),
+	}
+	statsCtx, ctx = stats.NewContext(ctx)
+	_, _, err = rc.handleHit(ctx, input, extents, 0)
+	require.NoError(t, err)
+	require.Equal(t, int64(30*time.Millisecond), statsCtx.Caches().Result.QueryLengthServed)
+}
+
 func TestResultsCacheMaxFreshness(t *testing.T) {
 	modelNow := model.Now()
 	for i, tc := range []struct {
@@ -603,3 +660,20 @@ type mockLimits struct {
 func (m mockLimits) MaxCacheFreshness(context.Context, string) time.Duration {
 	return m.maxCacheFreshness
 }
+
+type mockResultsCache struct{}
+
+func (m mockResultsCache) Store(context.Context, []string, [][]byte) error {
+	panic("not implemented")
+}
+func (m mockResultsCache) Fetch(context.Context, []string) ([]string, [][]byte, []string, error) {
+	panic("not implemented")
+}
+func (m mockResultsCache) Stop() {
+	panic("not implemented")
+}
+
+func (m mockResultsCache) GetCacheType() stats.CacheType {
+	// returning results cache since we do not support storing stats for "mock" cache
+	return stats.ResultCache
+}

pkg/util/marshal/legacy/marshal_test.go

@@ -108,7 +108,8 @@ var queryTests = []struct {
 						"bytesReceived": 0,
 						"bytesSent": 0,
 						"requests": 0,
-						"downloadTime": 0
+						"downloadTime": 0,
+						"queryLengthServed": 0
 					},
 					"index": {
 						"entriesFound": 0,
@@ -117,7 +118,8 @@ var queryTests = []struct {
 						"bytesReceived": 0,
 						"bytesSent": 0,
 						"requests": 0,
-						"downloadTime": 0
+						"downloadTime": 0,
+						"queryLengthServed": 0
 					},
 					"statsResult": {
 						"entriesFound": 0,
@@ -126,7 +128,8 @@ var queryTests = []struct {
 						"bytesReceived": 0,
 						"bytesSent": 0,
 						"requests": 0,
-						"downloadTime": 0
+						"downloadTime": 0,
+						"queryLengthServed": 0
 					},
 					"seriesResult": {
 						"entriesFound": 0,
@@ -135,7 +138,8 @@ var queryTests = []struct {
 						"bytesReceived": 0,
 						"bytesSent": 0,
 						"requests": 0,
-						"downloadTime": 0
+						"downloadTime": 0,
+						"queryLengthServed": 0
 					},
 					"labelResult": {
 						"entriesFound": 0,
@@ -144,7 +148,8 @@ var queryTests = []struct {
 						"bytesReceived": 0,
 						"bytesSent": 0,
 						"requests": 0,
-						"downloadTime": 0
+						"downloadTime": 0,
+						"queryLengthServed": 0
 					},
 					"volumeResult": {
 						"entriesFound": 0,
@@ -153,7 +158,8 @@ var queryTests = []struct {
 						"bytesReceived": 0,
 						"bytesSent": 0,
 						"requests": 0,
-						"downloadTime": 0
+						"downloadTime": 0,
+						"queryLengthServed": 0
 					},
 					"result": {
 						"entriesFound": 0,
@@ -162,7 +168,8 @@ var queryTests = []struct {
 						"bytesReceived": 0,
 						"bytesSent": 0,
 						"requests": 0,
-						"downloadTime": 0
+						"downloadTime": 0,
+						"queryLengthServed": 0
 					}
 				},
 				"summary": {

pkg/util/marshal/marshal_test.go

@@ -76,7 +76,8 @@ const emptyStats = `{
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"index": {
 			"entriesFound": 0,
@@ -85,7 +86,8 @@ const emptyStats = `{
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"statsResult": {
 			"entriesFound": 0,
@@ -94,7 +96,8 @@ const emptyStats = `{
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"seriesResult": {
 			"entriesFound": 0,
@@ -103,7 +106,8 @@ const emptyStats = `{
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"labelResult": {
 			"entriesFound": 0,
@@ -112,7 +116,8 @@ const emptyStats = `{
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"volumeResult": {
 			"entriesFound": 0,
@@ -121,7 +126,8 @@ const emptyStats = `{
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		},
 		"result": {
 			"entriesFound": 0,
@@ -130,7 +136,8 @@ const emptyStats = `{
 			"bytesReceived": 0,
 			"bytesSent": 0,
 			"requests": 0,
-			"downloadTime": 0
+			"downloadTime": 0,
+			"queryLengthServed": 0
 		}
 	},
 	"summary": {