From f1e1d20ac46407118db622077e800fb17ee0fca3 Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@victoriametrics.com>
Date: Fri, 8 Oct 2021 19:53:50 +0300
Subject: [PATCH] app/vmselect/promql: consistently return the same set of time
 series from `limitk()` function

This is the expected behaviour by most users.
---
 app/vmselect/promql/aggr.go      | 5 +++++
 app/vmselect/promql/exec_test.go | 2 +-
 docs/CHANGELOG.md                | 4 +++-
 docs/MetricsQL.md                | 2 +-
 4 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/app/vmselect/promql/aggr.go b/app/vmselect/promql/aggr.go
index 458061959f..ba908e6b89 100644
--- a/app/vmselect/promql/aggr.go
+++ b/app/vmselect/promql/aggr.go
@@ -1010,6 +1010,11 @@ func aggrFuncLimitK(afa *aggrFuncArg) ([]*timeseries, error) {
 		}
 	}
 	afe := func(tss []*timeseries, modifier *metricsql.ModifierExpr) []*timeseries {
+		// Sort series by metricName in order to get consistent set of output series
+		// across multiple calls to limitk() function.
+		sort.Slice(tss, func(i, j int) bool {
+			return metricNameLess(&tss[i].MetricName, &tss[j].MetricName)
+		})
 		if len(tss) > maxK {
 			tss = tss[:maxK]
 		}
diff --git a/app/vmselect/promql/exec_test.go b/app/vmselect/promql/exec_test.go
index 6c7df0b1a5..a90dd7f7bb 100644
--- a/app/vmselect/promql/exec_test.go
+++ b/app/vmselect/promql/exec_test.go
@@ -5003,7 +5003,7 @@ func TestExecSuccess(t *testing.T) {
 	})
 	t.Run(`limitk(1)`, func(t *testing.T) {
 		t.Parallel()
-		q := `limitk(1, label_set(10, "foo", "bar") or label_set(time()/150, "baz", "sss"))`
+		q := `limitk(1, label_set(10, "foo", "bar") or label_set(time()/150, "xbaz", "sss"))`
 		r1 := netstorage.Result{
 			MetricName: metricNameExpected,
 			Values:     []float64{10, 10, 10, 10, 10, 10},
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 73447c7cbf..f494b30ffd 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -6,7 +6,9 @@ sort: 15
 
 ## tip
 
-BUGFIX: vmstorage: fix `unaligned 64-bit atomic operation` panic on 32-bit architectures (arm and 386). The panic has been introduced in v1.67.0
+* FEATURE: consistently return the same set of time series from [limitk](https://docs.victoriametrics.com/MetricsQL.html#limitk) function. This improves the usability of periodically refreshed graphs.
+
+* BUGFIX: vmstorage: fix `unaligned 64-bit atomic operation` panic on 32-bit architectures (arm and 386). The panic has been introduced in v1.67.0
 
 
 ## [v1.67.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.67.0)
diff --git a/docs/MetricsQL.md b/docs/MetricsQL.md
index e700f951c8..2700443dbd 100644
--- a/docs/MetricsQL.md
+++ b/docs/MetricsQL.md
@@ -762,7 +762,7 @@ See also [implicit query conversions](#implicit-query-conversions).
 
 #### limitk
 
-`limitk(k, q) by (group_labels)` returns up to `k` time series per each `group_labels` out of time series returned by `q`. The returned set of time series can change with each call.
+`limitk(k, q) by (group_labels)` returns up to `k` time series per each `group_labels` out of time series returned by `q`. The returned set of time series remain the same across calls.
 
 #### mad