MQE: fix issue where subqueries could return series with no points

CHANGELOG.md

@@ -19,7 +19,7 @@
 * [CHANGE] Ingester: remove experimental flags `-ingest-storage.kafka.ongoing-records-per-fetch` and `-ingest-storage.kafka.startup-records-per-fetch`. They are removed in favour of `-ingest-storage.kafka.max-buffered-bytes`. #9906
 * [CHANGE] Ingester: Replace `cortex_discarded_samples_total` label from `sample-out-of-bounds` to `sample-timestamp-too-old`. #9885
 * [CHANGE] Ruler: the `/prometheus/config/v1/rules` does not return an error anymore if a rule group is missing in the object storage after been successfully returned by listing the storage, because it could have been deleted in the meanwhile. #9936
-* [FEATURE] Querier: add experimental streaming PromQL engine, enabled with `-querier.query-engine=mimir`. #9367 #9368 #9398 #9399 #9403 #9417 #9418 #9419 #9420 #9482 #9504 #9505 #9507 #9518 #9531 #9532 #9533 #9553 #9558 #9588 #9589 #9639 #9641 #9642 #9651 #9664 #9681 #9717 #9719 #9724 #9874
+* [FEATURE] Querier: add experimental streaming PromQL engine, enabled with `-querier.query-engine=mimir`. #9367 #9368 #9398 #9399 #9403 #9417 #9418 #9419 #9420 #9482 #9504 #9505 #9507 #9518 #9531 #9532 #9533 #9553 #9558 #9588 #9589 #9639 #9641 #9642 #9651 #9664 #9681 #9717 #9719 #9724 #9874 #9998
 * [FEATURE] Distributor: Add support for `lz4` OTLP compression. #9763
 * [FEATURE] Query-frontend: added experimental configuration options `query-frontend.cache-errors` and `query-frontend.results-cache-ttl-for-errors` to allow non-transient responses to be cached. When set to `true` error responses from hitting limits or bad data are cached for a short TTL. #9028
 * [FEATURE] Query-frontend: add middleware to control access to specific PromQL experimental functions on a per-tenant basis. #9798

pkg/streamingpromql/engine_test.go

@@ -904,6 +904,22 @@ func TestSubqueries(t *testing.T) {
 			},
 			Start: time.Unix(10, 0),
 		},
+		{
+			// A query where SeriesMetadata returns some series but evaluates to no samples should not return anything.
+			Query: `(metric > Inf)[20s:10s]`,
+			Start: time.Unix(30, 0),
+			Result: promql.Result{
+				Value: promql.Matrix{},
+			},
+		},
+		{
+			// A nested subquery with the same properties as above.
+			Query: `last_over_time((metric > Inf)[20s:10s])[30s:5s]`,
+			Start: time.Unix(30, 0),
+			Result: promql.Result{
+				Value: promql.Matrix{},
+			},
+		},
 		{
 			Query: "metric[20s:5s]",
 			Result: promql.Result{

pkg/streamingpromql/query.go

@@ -740,6 +740,12 @@ func (q *Query) populateMatrixFromRangeVectorOperator(ctx context.Context, o typ
 			return nil, err
 		}
 
+		if len(floats) == 0 && len(histograms) == 0 {
+			types.FPointSlicePool.Put(floats, q.memoryConsumptionTracker)
+			types.HPointSlicePool.Put(histograms, q.memoryConsumptionTracker)
+			continue
+		}
+
 		m = append(m, promql.Series{
 			Metric:     s.Labels,
 			Floats:     floats,

pkg/streamingpromql/testdata/ours/subqueries.test

@@ -127,3 +127,9 @@ eval range from 0 to 4m step 20s sum_over_time(sum_over_time(metric[2m:30s])[3m:
 
 eval range from 0 to 4m step 3m sum_over_time(sum_over_time(sum_over_time(metric[2m:30s])[3m:15s])[4m:20s])
   {} 0 86
+
+eval range from 0 to 4m step 15s last_over_time((metric > Inf)[20s:10s])
+  # Should produce no results.
+
+eval instant at 3m last_over_time((metric > Inf)[20s:10s])
+  # Should produce no results.