From 43dc23afe77db51070736eafb98fa87cd7ebd707 Mon Sep 17 00:00:00 2001
From: Ganesh Vernekar <ganesh.vernekar@reddit.com>
Date: Fri, 23 Jan 2026 18:02:45 -0800
Subject: [PATCH] tsdb: Clear stale series from the Head during WAL replay

Signed-off-by: Ganesh Vernekar <ganesh.vernekar@reddit.com>
---
 tsdb/head_wal.go | 35 +++++++++++++++++++++++++++++++----
 1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/tsdb/head_wal.go b/tsdb/head_wal.go
index bbcad9d855..b323f0dbf6 100644
--- a/tsdb/head_wal.go
+++ b/tsdb/head_wal.go
@@ -308,7 +308,21 @@ Outer:
 			}
 			h.wlReplaySamplesPool.Put(v)
 		case []tombstones.Stone:
+			// Tombstone records will be fairly rare, so not trying to optimise the allocations here.
+			deleteSeriesShards := make([][]chunks.HeadSeriesRef, concurrency)
 			for _, s := range v {
+				if len(s.Intervals) == 1 && s.Intervals[0].Mint == math.MinInt64 && s.Intervals[0].Maxt == math.MaxInt64 {
+					// This series was fully deleted at this point. This record is only done for stale series at the moment.
+					mod := uint64(s.Ref) % uint64(concurrency)
+					deleteSeriesShards[mod] = append(deleteSeriesShards[mod], chunks.HeadSeriesRef(s.Ref))
+
+					// If the series is with a different reference, try deleting that.
+					if r, ok := multiRef[chunks.HeadSeriesRef(s.Ref)]; ok {
+						mod := uint64(r) % uint64(concurrency)
+						deleteSeriesShards[mod] = append(deleteSeriesShards[mod], r)
+					}
+					continue
+				}
 				for _, itv := range s.Intervals {
 					if itv.Maxt < h.minValidTime.Load() {
 						continue
@@ -326,6 +340,14 @@ Outer:
 					h.tombstones.AddInterval(s.Ref, itv)
 				}
 			}
+
+			for i := range concurrency {
+				if len(deleteSeriesShards[i]) > 0 {
+					processors[i].input <- walSubsetProcessorInputItem{deletedSeriesRefs: deleteSeriesShards[i]}
+					deleteSeriesShards[i] = nil
+				}
+			}
+
 			h.wlReplaytStonesPool.Put(v)
 		case []record.RefExemplar:
 			for _, e := range v {
@@ -558,10 +580,11 @@ type walSubsetProcessor struct {
 }
 
 type walSubsetProcessorInputItem struct {
-	samples          []record.RefSample
-	histogramSamples []histogramRecord
-	existingSeries   *memSeries
-	walSeriesRef     chunks.HeadSeriesRef
+	samples           []record.RefSample
+	histogramSamples  []histogramRecord
+	existingSeries    *memSeries
+	walSeriesRef      chunks.HeadSeriesRef
+	deletedSeriesRefs []chunks.HeadSeriesRef
 }
 
 func (wp *walSubsetProcessor) setup() {
@@ -712,6 +735,10 @@ func (wp *walSubsetProcessor) processWALSamples(h *Head, mmappedChunks, oooMmapp
 		case wp.histogramsOutput <- in.histogramSamples:
 		default:
 		}
+
+		if len(in.deletedSeriesRefs) > 0 {
+			h.deleteSeriesByID(in.deletedSeriesRefs)
+		}
 	}
 	h.updateMinMaxTime(mint, maxt)