lib/logstorage: fix topk wrong ordering with mixed _time sources

docs/victorialogs/CHANGELOG.md

@@ -26,6 +26,7 @@ according to the following docs:
 * FEATURE: [alerts](https://github.com/VictoriaMetrics/VictoriaLogs/blob/master/deployment/docker/rules): add new alerting rules `PersistentQueueRunsOutOfSpaceIn12Hours` and `PersistentQueueRunsOutOfSpaceIn4Hours` for `vlagent` persistent queue capacity. These alerts help users to take proactive actions before `vlagent` starts dropping logs due to insufficient persistent queue space. See [#10193](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/10193)
 * FEATURE: [web UI](https://docs.victoriametrics.com/victorialogs/querying/#web-ui): remove the `Date format` setting and always display timestamps with nanosecond precision. See [#1161](https://github.com/VictoriaMetrics/VictoriaLogs/issues/1161).
 
+* BUGFIX: [LogsQL](https://docs.victoriametrics.com/victorialogs/logsql/): fix [`sort by (_time) limit N`](https://docs.victoriametrics.com/victorialogs/logsql/#sort-pipe) returning logs out of order when the query pipeline included pipes like [`unpack_json`](https://docs.victoriametrics.com/victorialogs/logsql/#unpack_json-pipe) that overwrite `_time`. See [#1360](https://github.com/VictoriaMetrics/VictoriaLogs/issues/1360).
 * BUGFIX: [vlagent](https://docs.victoriametrics.com/victorialogs/vlagent/): hide sensitive values passed via `-remoteWrite.proxyURL` in `/metrics`, `/flags`, and startup logs. Previously these values could be exposed in plain text. See [#1320](https://github.com/VictoriaMetrics/VictoriaLogs/pull/1320).
 * BUGFIX: [web UI](https://docs.victoriametrics.com/victorialogs/querying/#web-ui): sanitize markdown URLs in logs rendered with `markdown parsing` enabled, allowing only `http`, `https`, `mailto`, and `tel` schemes for active links and images. See [#1313](https://github.com/VictoriaMetrics/VictoriaLogs/pull/1313).
 * BUGFIX: [web UI](https://docs.victoriametrics.com/victorialogs/querying/#web-ui): improve context view highlight visibility in dark theme. The selected log entry is now highlighted with a more visible blue tint instead of barely visible gray background. See [#1196](https://github.com/VictoriaMetrics/VictoriaLogs/issues/1196).

lib/logstorage/pipe_sort_topk.go

@@ -679,7 +679,7 @@ func topkLess(ps *pipeSort, a, b *pipeTopkRow) bool {
 			bb.B = marshalTimestampRFC3339NanoString(bb.B[:0], a.timestamp)
 			vA = bytesutil.ToUnsafeString(bb.B)
 		} else if isTimeB[i] {
-			bb.B = marshalTimestampRFC3339NanoString(bb.B[:0], a.timestamp)
+			bb.B = marshalTimestampRFC3339NanoString(bb.B[:0], b.timestamp)
 			vB = bytesutil.ToUnsafeString(bb.B)
 		}
 

lib/logstorage/pipe_sort_topk_test.go

@@ -4,6 +4,64 @@ import (
 	"testing"
 )
 
+func TestTopkLess(t *testing.T) {
+	parseTimestamp := func(s string) int64 {
+		t.Helper()
+
+		timestamp, ok := TryParseTimestampRFC3339Nano(s)
+		if !ok {
+			t.Fatalf("cannot parse timestamp %q", s)
+		}
+		return timestamp
+	}
+
+	ps := &pipeSort{
+		byFields: []*bySortField{
+			{name: "_time"},
+		},
+	}
+	psDesc := &pipeSort{
+		byFields: []*bySortField{
+			{name: "_time"},
+		},
+		isDesc: true,
+	}
+
+	stringRow := func(s string) *pipeTopkRow {
+		return &pipeTopkRow{
+			byColumns:       []string{s},
+			byColumnsIsTime: []bool{false},
+		}
+	}
+	timeRow := func(s string) *pipeTopkRow {
+		return &pipeTopkRow{
+			byColumns:       []string{""},
+			byColumnsIsTime: []bool{true},
+			timestamp:       parseTimestamp(s),
+		}
+	}
+	f := func(ps *pipeSort, a, b *pipeTopkRow, resultExpected bool) {
+		t.Helper()
+
+		result := topkLess(ps, a, b)
+		if result != resultExpected {
+			t.Fatalf("unexpected result for topkLess(%#v, %#v); got %v; want %v", a, b, result, resultExpected)
+		}
+	}
+
+	// string time is smaller than real time
+	f(ps, stringRow("2026-04-25T10:00:54Z"), timeRow("2026-04-25T10:01:54Z"), true)
+	f(ps, timeRow("2026-04-25T10:01:54Z"), stringRow("2026-04-25T10:00:54Z"), false)
+
+	// real time is smaller than string time
+	f(ps, timeRow("2026-04-25T10:00:54Z"), stringRow("2026-04-25T10:01:54Z"), true)
+	f(ps, stringRow("2026-04-25T10:01:54Z"), timeRow("2026-04-25T10:00:54Z"), false)
+
+	// string time vs real time with descending sort
+	f(psDesc, stringRow("2026-04-25T10:00:54Z"), timeRow("2026-04-25T10:01:54Z"), false)
+	f(psDesc, timeRow("2026-04-25T10:01:54Z"), stringRow("2026-04-25T10:00:54Z"), true)
+}
+
 func TestLessString(t *testing.T) {
 	f := func(a, b string, resultExpected bool) {
 		t.Helper()