sort pgids on file open only if they aren't sorted

freelist.go

@@ -172,7 +172,9 @@ func (f *freelist) read(p *page) {
 	copy(f.ids, ids)
 
 	// Make sure they're sorted.
-	sort.Sort(pgids(f.ids))
+	if !pgids(f.ids).isSorted() {
+		sort.Sort(pgids(f.ids))
+	}
 
 	// Rebuild the page cache.
 	f.reindex()

page.go

@@ -133,6 +133,17 @@ type pgids []pgid
 func (s pgids) Len() int           { return len(s) }
 func (s pgids) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
 func (s pgids) Less(i, j int) bool { return s[i] < s[j] }
+func (s pgids) isSorted() bool {
+	if len(s) == 0 {
+		return true
+	}
+	for i, v := range s[:len(s)-1] {
+		if s[i+1] < v {
+			return false
+		}
+	}
+	return true
+}
 
 // merge returns the sorted union of a and b.
 func (a pgids) merge(b pgids) pgids {