Improve last msg lookup (KV Get) when subject is a literal subject.

Signed-off-by: Derek Collison <derek@nats.io>

server/filestore.go

@@ -4665,7 +4665,16 @@ func (fs *fileStore) loadLast(subj string, sm *StoreMsg) (lsm *StoreMsg, err err
 			mb.mu.Unlock()
 			return nil, err
 		}
-		_, _, l := mb.filteredPendingLocked(subj, wc, mb.first.seq)
+		var l uint64
+		// Optimize if subject is not a wildcard.
+		if !wc {
+			if ss := mb.fss[subj]; ss != nil {
+				l = ss.Last
+			}
+		}
+		if l == 0 {
+			_, _, l = mb.filteredPendingLocked(subj, wc, mb.first.seq)
+		}
 		if l > 0 {
 			if mb.cacheNotLoaded() {
 				if err := mb.loadMsgsWithLock(); err != nil {

server/memstore.go

@@ -864,6 +864,10 @@ func (ms *memStore) LoadLastMsg(subject string, smp *StoreMsg) (*StoreMsg, error
 
 	if subject == _EMPTY_ || subject == fwcs {
 		sm, ok = ms.msgs[ms.state.LastSeq]
+	} else if subjectIsLiteral(subject) {
+		if ss := ms.fss[subject]; ss != nil && ss.Msgs > 0 {
+			sm, ok = ms.msgs[ss.Last]
+		}
 	} else if ss := ms.filteredStateLocked(1, subject, true); ss.Msgs > 0 {
 		sm, ok = ms.msgs[ss.Last]
 	}
@@ -895,8 +899,8 @@ func (ms *memStore) LoadNextMsg(filter string, wc bool, start uint64, smp *Store
 
 	isAll := filter == _EMPTY_ || filter == fwcs
 
-	// Skip scan of mb.fss is number of messages in the block are less than
-	// 1/2 the number of subjects in mb.fss. Or we have a wc and lots of fss entries.
+	// Skip scan of ms.fss is number of messages in the block are less than
+	// 1/2 the number of subjects in ms.fss. Or we have a wc and lots of fss entries.
 	const linearScanMaxFSS = 256
 	doLinearScan := isAll || 2*int(ms.state.LastSeq-start) < len(ms.fss) || (wc && len(ms.fss) > linearScanMaxFSS)