Make sure if we have lots of subjects to fallback to subjects state (#…

…4713)

This is optimization for last change, the get last works but not if
number of subjects is very large, for that fall back to subjects state.

Signed-off-by: Derek Collison <[email protected]>

server/consumer.go

@@ -4687,8 +4687,9 @@ 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 smv StoreMsg
 				var filters []string
 				if o.subjf == nil {
 					filters = append(filters, o.cfg.FilterSubject)
@@ -4698,9 +4699,16 @@ func (o *consumer) selectStartingSeqNo() {
 					}
 				}
 				for _, filter := range filters {
-					for subj := range o.mset.store.SubjectsTotals(filter) {
-						if sm, err := o.mset.store.LoadLastMsg(subj, &smv); err == nil {
-							lss.seqs = append(lss.seqs, sm.seq)
+					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)
 						}
 					}
 				}