From cc9072ad649518bc77f818758f7a7a2f64219ab8 Mon Sep 17 00:00:00 2001
From: Patrick Oyarzun <patrick.oyarzun@grafana.com>
Date: Mon, 17 Apr 2023 19:58:56 -0500
Subject: [PATCH 1/2] Apply matchers when fetching label values

---
 tsdb/querier.go            | 18 ++++++++++++++++++
 tsdb/querier_bench_test.go |  1 +
 2 files changed, 19 insertions(+)

diff --git a/tsdb/querier.go b/tsdb/querier.go
index aed4952ac0..a8a18164aa 100644
--- a/tsdb/querier.go
+++ b/tsdb/querier.go
@@ -338,6 +338,24 @@ func labelValuesWithMatchers(r IndexReader, name string, matchers ...*labels.Mat
 	if err != nil {
 		return nil, errors.Wrapf(err, "fetching values of label %s", name)
 	}
+
+	// If we have a matcher for the label name, we can filter out values that don't match
+	// before we fetch postings. This is especially useful for labels with many values.
+	// e.g. __name__ with a selector like {__name__="xyz"}
+	for _, m := range matchers {
+		if m.Name != name {
+			continue
+		}
+
+		for i := len(allValues) - 1; i >= 0; i-- {
+			if m.Matches(allValues[i]) {
+				continue
+			}
+
+			allValues = append(allValues[:i], allValues[i+1:]...)
+		}
+	}
+
 	valuesPostings := make([]index.Postings, len(allValues))
 	for i, value := range allValues {
 		valuesPostings[i], err = r.Postings(name, value)
diff --git a/tsdb/querier_bench_test.go b/tsdb/querier_bench_test.go
index bd8f7131cf..741301dca6 100644
--- a/tsdb/querier_bench_test.go
+++ b/tsdb/querier_bench_test.go
@@ -182,6 +182,7 @@ func benchmarkLabelValuesWithMatchers(b *testing.B, ir IndexReader) {
 		labelName string
 		matchers  []*labels.Matcher
 	}{
+		{`i with i="1"`, "i", []*labels.Matcher{i1}},
 		// i has 100k values.
 		{`i with n="1"`, "i", []*labels.Matcher{n1}},
 		{`i with n="^.+$"`, "i", []*labels.Matcher{nPlus}},

From e52ce3e361239e5dfe6045787de9a161319a675b Mon Sep 17 00:00:00 2001
From: Patrick Oyarzun <patrick.oyarzun@grafana.com>
Date: Tue, 18 Apr 2023 15:50:05 -0500
Subject: [PATCH 2/2] Avoid extra copying of label values

---
 tsdb/querier.go | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/tsdb/querier.go b/tsdb/querier.go
index a8a18164aa..1e6a817afb 100644
--- a/tsdb/querier.go
+++ b/tsdb/querier.go
@@ -347,13 +347,15 @@ func labelValuesWithMatchers(r IndexReader, name string, matchers ...*labels.Mat
 			continue
 		}
 
-		for i := len(allValues) - 1; i >= 0; i-- {
-			if m.Matches(allValues[i]) {
-				continue
+		// re-use the allValues slice to avoid allocations
+		// this is safe because the iteration is always ahead of the append
+		filteredValues := allValues[:0]
+		for _, v := range allValues {
+			if m.Matches(v) {
+				filteredValues = append(filteredValues, v)
 			}
-
-			allValues = append(allValues[:i], allValues[i+1:]...)
 		}
+		allValues = filteredValues
 	}
 
 	valuesPostings := make([]index.Postings, len(allValues))