Query ingesters only for GetLabelNames and GetLabelValues if start time parameter is not specified

CHANGELOG.md

@@ -6,6 +6,7 @@
 * [FEATURE] Querier/Ruler: Add `query_partial_data` and `rules_partial_data` limits to allow queries/rules to be evaluated with data from a single zone, if other zones are not available. #6526
 * [FEATURE] Update prometheus alertmanager version to v0.28.0 and add new integration msteamsv2, jira, and rocketchat. #6590
 * [FEATURE] Ingester: Add a `-ingester.enable-ooo-native-histograms` flag to enable out-of-order native histogram ingestion per tenant. It only takes effect when `-blocks-storage.tsdb.enable-native-histograms=true` and `-ingester.out-of-order-time-window` > 0. It is applied after the restart if it is changed at runtime through the runtime config. #6626
+* [ENHANCEMENT] Querier: limit label APIs to query only ingesters if `start` param is not been specified. #6618
 * [ENHANCEMENT] Alertmanager: Add new limits `-alertmanager.max-silences-count` and `-alertmanager.max-silences-size-bytes` for limiting silences per tenant. #6605
 * [ENHANCEMENT] Update prometheus version to v3.1.0. #6583
 * [ENHANCEMENT] Add `compactor.auto-forget-delay` for compactor to auto forget compactors after X minutes without heartbeat. #6533

docs/api/_index.md

@@ -390,7 +390,7 @@ GET,POST <prometheus-http-prefix>/api/v1/labels
 GET,POST <legacy-http-prefix>/api/v1/labels
 ```
 
-Get label names of ingested series. Starting from release v1.18.0, Cortex by default honors the `start` and `end` request parameters and fetches label names from either ingester, store gateway or both.
+Get label names of ingested series. Starting from release v1.18.0, Cortex by default honors the `start` and `end` request parameters and fetches label names from either ingester, store gateway or both. The special case is that if `start` param is not specified, Cortex currently fetches labels from data stored in the ingesters.
 
 _For more information, please check out the Prometheus [get label names](https://prometheus.io/docs/prometheus/latest/querying/api/#getting-label-names) documentation._
 
@@ -405,7 +405,7 @@ GET <prometheus-http-prefix>/api/v1/label/{name}/values
 GET <legacy-http-prefix>/api/v1/label/{name}/values
 ```
 
-Get label values for a given label name. Starting from release v1.18.0, Cortex by default honors the `start` and `end` request parameters and fetches label values from either ingester, store gateway or both.
+Get label values for a given label name. Starting from release v1.18.0, Cortex by default honors the `start` and `end` request parameters and fetches label values from either ingester, store gateway or both. The special case is that if `start` param is not specified, Cortex currently fetches label values from data stored in the ingesters.
 
 _For more information, please check out the Prometheus [get label values](https://prometheus.io/docs/prometheus/latest/querying/api/#querying-label-values) documentation._
 

pkg/querier/querier.go

@@ -431,7 +431,7 @@ func (q querier) Select(ctx context.Context, sortSeries bool, sp *storage.Select
 
 // LabelValues implements storage.Querier.
 func (q querier) LabelValues(ctx context.Context, name string, hints *storage.LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
-	ctx, stats, userID, mint, maxt, _, queriers, err := q.setupFromCtx(ctx)
+	ctx, stats, userID, mint, maxt, metadataQuerier, queriers, err := q.setupFromCtx(ctx)
 	if err == errEmptyTimeRange {
 		return nil, nil, nil
 	} else if err != nil {
@@ -442,6 +442,12 @@ func (q querier) LabelValues(ctx context.Context, name string, hints *storage.La
 		stats.AddQueryStorageWallTime(time.Since(startT))
 	}()
 
+	// For label values queries without specifying the start time, we prefer to
+	// only query ingesters and not to query maxQueryLength to avoid OOM kill.
+	if mint == 0 {
+		return metadataQuerier.LabelValues(ctx, name, hints, matchers...)
+	}
+
 	startTime := model.Time(mint)
 	endTime := model.Time(maxt)
 
@@ -494,7 +500,7 @@ func (q querier) LabelValues(ctx context.Context, name string, hints *storage.La
 }
 
 func (q querier) LabelNames(ctx context.Context, hints *storage.LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
-	ctx, stats, userID, mint, maxt, _, queriers, err := q.setupFromCtx(ctx)
+	ctx, stats, userID, mint, maxt, metadataQuerier, queriers, err := q.setupFromCtx(ctx)
 	if err == errEmptyTimeRange {
 		return nil, nil, nil
 	} else if err != nil {
@@ -505,6 +511,12 @@ func (q querier) LabelNames(ctx context.Context, hints *storage.LabelHints, matc
 		stats.AddQueryStorageWallTime(time.Since(startT))
 	}()
 
+	// For label names queries without specifying the start time, we prefer to
+	// only query ingesters and not to query maxQueryLength to avoid OOM kill.
+	if mint == 0 {
+		return metadataQuerier.LabelNames(ctx, hints, matchers...)
+	}
+
 	startTime := model.Time(mint)
 	endTime := model.Time(maxt)