From c999177e1d53f47e4840923b72ba49ee2c62110d Mon Sep 17 00:00:00 2001
From: Derek Collison <derek@nats.io>
Date: Thu, 26 Oct 2023 19:15:35 -0700
Subject: [PATCH 1/2] If we have deliver last by subject and max msgs per
 subject of 1, we can short circuit to normal consumer.

Signed-off-by: Derek Collison <derek@nats.io>
---
 server/consumer.go | 70 +++++++++++++++++++++++++---------------------
 1 file changed, 38 insertions(+), 32 deletions(-)

diff --git a/server/consumer.go b/server/consumer.go
index 3b7fe4dd63b..1f96e772e8c 100644
--- a/server/consumer.go
+++ b/server/consumer.go
@@ -4687,44 +4687,50 @@ func (o *consumer) selectStartingSeqNo() {
 					}
 				}
 			} else if o.cfg.DeliverPolicy == DeliverLastPerSubject {
-				// A threshold for when we switch from get last msg to subjects state.
-				const numSubjectsThresh = 256
-				lss := &lastSeqSkipList{resume: state.LastSeq}
-				var filters []string
-				if o.subjf == nil {
-					filters = append(filters, o.cfg.FilterSubject)
+				// If our parent stream is set to max msgs per subject of 1 this is just
+				// a normal consumer at this point. We can avoid any heavy lifting.
+				if o.mset.cfg.MaxMsgsPer == 1 {
+					o.sseq = state.FirstSeq
 				} else {
-					for _, filter := range o.subjf {
-						filters = append(filters, filter.subject)
+					// A threshold for when we switch from get last msg to subjects state.
+					const numSubjectsThresh = 256
+					lss := &lastSeqSkipList{resume: state.LastSeq}
+					var filters []string
+					if o.subjf == nil {
+						filters = append(filters, o.cfg.FilterSubject)
+					} else {
+						for _, filter := range o.subjf {
+							filters = append(filters, filter.subject)
+						}
 					}
-				}
-				for _, filter := range filters {
-					if st := o.mset.store.SubjectsTotals(filter); len(st) < numSubjectsThresh {
-						var smv StoreMsg
-						for subj := range st {
-							if sm, err := o.mset.store.LoadLastMsg(subj, &smv); err == nil {
-								lss.seqs = append(lss.seqs, sm.seq)
+					for _, filter := range filters {
+						if st := o.mset.store.SubjectsTotals(filter); len(st) < numSubjectsThresh {
+							var smv StoreMsg
+							for subj := range st {
+								if sm, err := o.mset.store.LoadLastMsg(subj, &smv); err == nil {
+									lss.seqs = append(lss.seqs, sm.seq)
+								}
+							}
+						} else if mss := o.mset.store.SubjectsState(filter); len(mss) > 0 {
+							for _, ss := range mss {
+								lss.seqs = append(lss.seqs, ss.Last)
 							}
-						}
-					} else if mss := o.mset.store.SubjectsState(filter); len(mss) > 0 {
-						for _, ss := range mss {
-							lss.seqs = append(lss.seqs, ss.Last)
 						}
 					}
+					// Sort the skip list if needed.
+					if len(lss.seqs) > 1 {
+						sort.Slice(lss.seqs, func(i, j int) bool {
+							return lss.seqs[j] > lss.seqs[i]
+						})
+					}
+					if len(lss.seqs) == 0 {
+						o.sseq = state.LastSeq
+					} else {
+						o.sseq = lss.seqs[0]
+					}
+					// Assign skip list.
+					o.lss = lss
 				}
-				// Sort the skip list if needed.
-				if len(lss.seqs) > 1 {
-					sort.Slice(lss.seqs, func(i, j int) bool {
-						return lss.seqs[j] > lss.seqs[i]
-					})
-				}
-				if len(lss.seqs) == 0 {
-					o.sseq = state.LastSeq
-				} else {
-					o.sseq = lss.seqs[0]
-				}
-				// Assign skip list.
-				o.lss = lss
 			} else if o.cfg.OptStartTime != nil {
 				// If we are here we are time based.
 				// TODO(dlc) - Once clustered can't rely on this.

From 85784a384b4c47c66306c044ccfb182611de4231 Mon Sep 17 00:00:00 2001
From: Derek Collison <derek@nats.io>
Date: Thu, 26 Oct 2023 19:27:41 -0700
Subject: [PATCH 2/2] Only load msgs if mb.fss has been evicted.

Signed-off-by: Derek Collison <derek@nats.io>
---
 server/filestore.go | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/server/filestore.go b/server/filestore.go
index 4fcbea4d759..3157e6784f4 100644
--- a/server/filestore.go
+++ b/server/filestore.go
@@ -2426,7 +2426,7 @@ func (fs *fileStore) SubjectsState(subject string) map[string]SimpleState {
 	fs.mu.RLock()
 	defer fs.mu.RUnlock()
 
-	if fs.state.Msgs == 0 {
+	if fs.state.Msgs == 0 || fs.noTrackSubjects() {
 		return nil
 	}
 
@@ -2454,7 +2454,7 @@ func (fs *fileStore) SubjectsState(subject string) map[string]SimpleState {
 
 		mb.mu.Lock()
 		var shouldExpire bool
-		if mb.cacheNotLoaded() {
+		if mb.fss == nil {
 			// Make sure we have fss loaded.
 			mb.loadMsgsWithLock()
 			shouldExpire = true