lib/storage: include last sample when query at the last millisecond of the day

One millisecond shouldn't be subtracted from the `tr.MaxTimestamp`, and
related test cases will be added

Fixes https://github.com/VictoriaMetrics/VictoriaMetrics/issues/9804

apptest/tests/special_query_regression_test.go

@@ -53,6 +53,7 @@ func testSpecialQueryRegression(tc *apptest.TestCase, sut apptest.PrometheusWrit
 	testMatchSeries(tc, sut)
 	testNegativeIncrease(tc, sut)
 	testInstantQueryWithOffsetUsingCache(tc, sut)
+	testQueryRangeEndAtFirstMillisecondOfDate(tc, sut)
 
 	// graphite
 	testComparisonNotInfNotNan(tc, sut)
@@ -334,6 +335,48 @@ func testInstantQueryWithOffsetUsingCache(tc *apptest.TestCase, sut apptest.Prom
 	})
 }
 
+func testQueryRangeEndAtFirstMillisecondOfDate(tc *apptest.TestCase, sut apptest.PrometheusWriteQuerier) {
+	t := tc.T()
+
+	// unexpected /api/v1/query_range response
+	// when the sample is at the last millisecond of a day, e.g. `2025-12-12 00:00:00`
+	// query_range with `End` at the last millisecond of that day may cause the time point to be missed.
+	// see https://github.com/VictoriaMetrics/VictoriaMetrics/issues/9804
+
+	// `End` should be inclusive according to https://prometheus.io/docs/prometheus/latest/querying/api/#range-queries
+
+	sut.PrometheusAPIV1ImportPrometheus(t, []string{
+		`foo_bar 7 1765497600000`, // 2025-12-12 00:00:00
+	}, apptest.QueryOpts{})
+	sut.ForceFlush(t)
+
+	tc.Assert(&apptest.AssertOptions{
+		Msg:        "unexpected /api/v1/query response",
+		DoNotRetry: true,
+		Got: func() any {
+			return sut.PrometheusAPIV1QueryRange(t, `foo_bar`, apptest.QueryOpts{
+				Start: "2025-12-11T20:00:00.000Z",
+				End:   "2025-12-12T00:00:00.000Z",
+				Step:  "1h",
+			})
+		},
+		Want: &apptest.PrometheusAPIV1QueryResponse{
+			Status: "success",
+			Data: &apptest.QueryData{
+				ResultType: "matrix",
+				Result: []*apptest.QueryResult{
+					{
+						Metric: map[string]string{"__name__": "foo_bar"},
+						Samples: []*apptest.Sample{
+							{Timestamp: 1765497600000, Value: 7},
+						},
+					},
+				},
+			},
+		},
+	})
+}
+
 func testComparisonNotInfNotNan(tc *apptest.TestCase, sut apptest.PrometheusWriteQuerier) {
 	t := tc.T()
 

docs/victoriametrics/changelog/CHANGELOG.md

@@ -50,6 +50,7 @@ See also [LTS releases](https://docs.victoriametrics.com/victoriametrics/lts-rel
 * BUGFIX: [vmsingle](https://docs.victoriametrics.com/victoriametrics/single-server-victoriametrics/) and `vmstorage` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/victoriametrics/cluster-victoriametrics/): `dateMetricIDCache` metrics were renamed to follow the naming pattern used for other caches. `vm_date_metric_id_cache_resets_total` becomes `vm_cache_resets_total{type="indexdb/date_metricID"}` and `vm_date_metric_id_cache_syncs_total` became `vm_cache_syncs_total{type="indexdb/date_metricID"}`. See PR [#10152](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/10152) for details.
 * BUGFIX: [vmsingle](https://docs.victoriametrics.com/victoriametrics/single-server-victoriametrics/) and `vmstorage` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/victoriametrics/cluster-victoriametrics/): fix performance degradation caused by redundant indexdb lookups at the start of each hour. See this issue [#10114](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/10114) for details.
 * BUGFIX: [vmauth](https://docs.victoriametrics.com/victoriametrics/vmauth/): verify backend network reachability with a TCP dial before marking it healthy. Previously, backends were auto-restored after `-failTimeout` even if the network was still unreachable, causing requests to hang repeatedly. `vmauth` now performs a 1s TCP dial check before returning a backend to the healthy pool. See [#9997](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/9997).
+* BUGFIX: [vmsingle](https://docs.victoriametrics.com/victoriametrics/single-server-victoriametrics/) and `vmstorage` in [VictoriaMetrics cluster](https://docs.victoriametrics.com/victoriametrics/cluster-victoriametrics/): fix potential datapoint lost in response when query at the last millisecond of the day. See issue [#9804](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/9804) for details.
 
 ## [v1.131.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.131.0)
 

lib/storage/storage_test.go

@@ -3183,7 +3183,7 @@ func TestStorageAdjustTimeRange(t *testing.T) {
 	// Time range is smaller than 40 days. When the -disablePerDayIndex flag is
 	// unset, the time range will not be adjusted. When the flag is set, the
 	// adjusted time range will be globalIndexTimeRange.
-	tr = TimeRange{10 * msecPerDay, 50 * msecPerDay}
+	tr = TimeRange{10 * msecPerDay, 50*msecPerDay - 1}
 	f(false, tr, tr)
 	f(true, tr, globalIndexTimeRange)
 
@@ -3202,7 +3202,7 @@ func TestStorageAdjustTimeRange(t *testing.T) {
 	// When the -disablePerDayIndex flag is unset, the time range will not be
 	// adjusted. When the flag is set, the adjusted time range will be
 	// globalIndexTimeRange.
-	tr = TimeRange{10 * msecPerDay, 51 * msecPerDay}
+	tr = TimeRange{10 * msecPerDay, 50 * msecPerDay}
 	f(false, tr, tr)
 	f(true, tr, globalIndexTimeRange)
 

lib/storage/time.go

@@ -43,10 +43,9 @@ var (
 // DateRange returns the date range for the given time range.
 func (tr *TimeRange) DateRange() (uint64, uint64) {
 	minDate := uint64(tr.MinTimestamp) / msecPerDay
-	// Max timestamp may point to the first millisecond of the next day. As the
-	// result, the returned date range will cover one more day than needed.
-	// Decrementing by 1 removes this extra day.
-	maxDate := uint64(tr.MaxTimestamp-1) / msecPerDay
+	// Sample at Max timestamp should be included because `End` is inclusive.
+	// According to https://prometheus.io/docs/prometheus/latest/querying/api/#range-queries
+	maxDate := uint64(tr.MaxTimestamp) / msecPerDay
 
 	// However, if both timestamps are the same and point to the beginning of
 	// the day, then maxDate will be smaller that the minDate. In this case

lib/storage/time_test.go

@@ -138,6 +138,16 @@ func TestTimeRangeDateRange(t *testing.T) {
 	// the same as min date.
 	tr = TimeRange{2*msecPerDay + 654, 1*msecPerDay + 321}
 	f(tr, 2, 2)
+
+	// MaxTimestamp is the last millisecond of the day.
+	// Max date should be the next date
+	tr = TimeRange{1*msecPerDay + 123, 2 * msecPerDay}
+	f(tr, 1, 2)
+
+	// MaxTimestamp is the first millisecond of the day.
+	// Max date should be the next date
+	tr = TimeRange{1*msecPerDay + 123, 2*msecPerDay + 1}
+	f(tr, 1, 2)
 }
 
 func TestDateToString(t *testing.T) {