diff --git a/lib/storage/index_db.go b/lib/storage/index_db.go
index 9eefb6e96..cf3eeffc4 100644
--- a/lib/storage/index_db.go
+++ b/lib/storage/index_db.go
@@ -264,8 +264,7 @@ func (db *indexDB) SetExtDB(extDB *indexDB) {
 	// Add deleted metricIDs from extDB to db.
 	if extDB != nil {
 		dmisExt := extDB.getDeletedMetricIDs()
-		metricIDs := dmisExt.AppendTo(nil)
-		db.updateDeletedMetricIDs(metricIDs)
+		db.updateDeletedMetricIDs(dmisExt)
 	}
 
 	db.extDBLock.Lock()
@@ -885,7 +884,11 @@ func (db *indexDB) DeleteTSIDs(tfss []*TagFilters) (int, error) {
 	deletedCount := len(metricIDs)
 
 	// atomically add deleted metricIDs to an inmemory map.
-	db.updateDeletedMetricIDs(metricIDs)
+	dmis := &uint64set.Set{}
+	for _, metricID := range metricIDs {
+		dmis.Add(metricID)
+	}
+	db.updateDeletedMetricIDs(dmis)
 
 	// Reset TagFilters -> TSIDS cache, since it may contain deleted TSIDs.
 	invalidateTagCache()
@@ -914,13 +917,11 @@ func (db *indexDB) setDeletedMetricIDs(dmis *uint64set.Set) {
 	db.deletedMetricIDs.Store(dmis)
 }
 
-func (db *indexDB) updateDeletedMetricIDs(metricIDs []uint64) {
+func (db *indexDB) updateDeletedMetricIDs(metricIDs *uint64set.Set) {
 	db.deletedMetricIDsUpdateLock.Lock()
 	dmisOld := db.getDeletedMetricIDs()
 	dmisNew := dmisOld.Clone()
-	for _, metricID := range metricIDs {
-		dmisNew.Add(metricID)
-	}
+	dmisNew.Union(metricIDs)
 	db.setDeletedMetricIDs(dmisNew)
 	db.deletedMetricIDsUpdateLock.Unlock()
 }
@@ -1584,9 +1585,7 @@ func (is *indexSearch) updateMetricIDsForTagFilters(metricIDs *uint64set.Set, tf
 		}
 		minMetricIDs = mIDs
 	}
-	for _, metricID := range minMetricIDs.AppendTo(nil) {
-		metricIDs.Add(metricID)
-	}
+	metricIDs.Union(minMetricIDs)
 	return nil
 }
 
@@ -1911,9 +1910,7 @@ func (is *indexSearch) getMetricIDsForRecentHours(tr TimeRange, maxMetrics int)
 			return nil, false
 		}
 		metricIDs := hmCurr.m.Clone()
-		for _, metricID := range hmPrev.m.AppendTo(nil) {
-			metricIDs.Add(metricID)
-		}
+		metricIDs.Union(hmPrev.m)
 		return metricIDs, true
 	}
 	return nil, false
diff --git a/lib/storage/storage.go b/lib/storage/storage.go
index 3339382b0..a446f7f0a 100644
--- a/lib/storage/storage.go
+++ b/lib/storage/storage.go
@@ -938,12 +938,10 @@ func (s *Storage) updateCurrHourMetricIDs() {
 		isFull = true
 	}
 	s.pendingHourMetricIDsLock.Lock()
-	newMetricIDs := s.pendingHourMetricIDs.AppendTo(nil)
+	newMetricIDs := s.pendingHourMetricIDs
 	s.pendingHourMetricIDs = &uint64set.Set{}
 	s.pendingHourMetricIDsLock.Unlock()
-	for _, metricID := range newMetricIDs {
-		m.Add(metricID)
-	}
+	m.Union(newMetricIDs)
 
 	hmNew := &hourMetricIDs{
 		m:      m,
diff --git a/lib/uint64set/uint64set.go b/lib/uint64set/uint64set.go
index 54cc96b87..285f1e527 100644
--- a/lib/uint64set/uint64set.go
+++ b/lib/uint64set/uint64set.go
@@ -126,6 +126,14 @@ func (s *Set) AppendTo(dst []uint64) []uint64 {
 	return dst
 }
 
+// Union adds all the items from a to s.
+func (s *Set) Union(a *Set) {
+	// TODO: optimize it
+	for _, x := range a.AppendTo(nil) {
+		s.Add(x)
+	}
+}
+
 type bucket32 struct {
 	hi      uint32
 	b16his  []uint16
diff --git a/lib/uint64set/uint64set_test.go b/lib/uint64set/uint64set_test.go
index 2b6733b60..65628c2e3 100644
--- a/lib/uint64set/uint64set_test.go
+++ b/lib/uint64set/uint64set_test.go
@@ -88,6 +88,22 @@ func testSetBasicOps(t *testing.T, itemsCount int) {
 		}
 	}
 
+	// Verify union
+	const unionOffset = 12345
+	var s1, s2 Set
+	for i := 0; i < itemsCount; i++ {
+		s1.Add(uint64(i) + offset)
+		s2.Add(uint64(i) + offset + unionOffset)
+	}
+	s1.Union(&s2)
+	expectedLen := 2 * itemsCount
+	if itemsCount > unionOffset {
+		expectedLen = itemsCount + unionOffset
+	}
+	if n := s1.Len(); n != expectedLen {
+		t.Fatalf("unexpected s1.Len() after union; got %d; want %d", n, expectedLen)
+	}
+
 	// Verify Del
 	for i := itemsCount / 2; i < itemsCount-itemsCount/4; i++ {
 		s.Del(uint64(i) + offset)