diff --git a/db.go b/db.go
index 93e2dfd1a..e6ec3bca0 100644
--- a/db.go
+++ b/db.go
@@ -466,18 +466,19 @@ func (db *DB) Appender() Appender {
 	// anyway. For now this, with combination of only having a single timestamp per batch,
 	// prevents opening more than one appender and hitting an unresolved deadlock (#11).
 	//
-	// // Only instantiate appender after returning the headmtx to avoid
-	// // questionable locking order.
-	// db.headmtx.RLock()
-	// app := db.appendable()
-	// db.headmtx.RUnlock()
 
-	// for _, b := range app {
-	// 	a.heads = append(a.heads, &metaAppender{
-	// 		meta: b.Meta(),
-	// 		app:  b.Appender().(*headAppender),
-	// 	})
-	// }
+	// Only instantiate appender after returning the headmtx to avoid
+	// questionable locking order.
+	db.headmtx.RLock()
+	app := db.appendable()
+	db.headmtx.RUnlock()
+
+	for _, b := range app {
+		a.heads = append(a.heads, &metaAppender{
+			meta: b.Meta(),
+			app:  b.Appender(),
+		})
+	}
 
 	return a
 }
@@ -556,26 +557,26 @@ func (a *dbAppender) appenderFor(t int64) (*metaAppender, error) {
 		a.db.headmtx.Unlock()
 
 		// XXX(fabxc): temporary workaround. See comment on instantiating DB.Appender.
-		for _, b := range newHeads {
-			// Only get appender for the block with the specific timestamp.
-			if t >= b.Meta().MaxTime {
-				continue
-			}
-			a.heads = append(a.heads, &metaAppender{
-				app:  b.Appender(),
-				meta: b.Meta(),
-			})
-			break
-		}
-
-		// // Instantiate appenders after returning headmtx to avoid questionable
-		// // locking order.
 		// for _, b := range newHeads {
+		// 	// Only get appender for the block with the specific timestamp.
+		// 	if t >= b.Meta().MaxTime {
+		// 		continue
+		// 	}
 		// 	a.heads = append(a.heads, &metaAppender{
 		// 		app:  b.Appender(),
 		// 		meta: b.Meta(),
 		// 	})
+		// 	break
 		// }
+
+		// Instantiate appenders after returning headmtx to avoid questionable
+		// locking order.
+		for _, b := range newHeads {
+			a.heads = append(a.heads, &metaAppender{
+				app:  b.Appender(),
+				meta: b.Meta(),
+			})
+		}
 	}
 	for i := len(a.heads) - 1; i >= 0; i-- {
 		if h := a.heads[i]; t >= h.meta.MinTime {
@@ -613,28 +614,37 @@ func (db *DB) ensureHead(t int64) error {
 }
 
 func (a *dbAppender) Commit() error {
-	var merr MultiError
+	defer a.db.mtx.RUnlock()
+
+	// Commits to partial appenders must be concurrent as concurrent appenders
+	// may have conflicting locks on head appenders.
+	// XXX(fabxc): is this a leaky abstraction? Should make an effort to catch a multi-error?
+	var g errgroup.Group
 
 	for _, h := range a.heads {
-		merr.Add(h.app.Commit())
+		g.Go(h.app.Commit)
 	}
-	a.db.mtx.RUnlock()
 
-	if merr.Err() == nil {
-		a.db.metrics.samplesAppended.Add(float64(a.samples))
+	if err := g.Wait(); err != nil {
+		return err
 	}
-	return merr.Err()
+	// XXX(fabxc): Push the metric down into head block to account properly
+	// for partial appends?
+	a.db.metrics.samplesAppended.Add(float64(a.samples))
+
+	return nil
 }
 
 func (a *dbAppender) Rollback() error {
-	var merr MultiError
+	defer a.db.mtx.RUnlock()
+
+	var g errgroup.Group
 
 	for _, h := range a.heads {
-		merr.Add(h.app.Rollback())
+		g.Go(h.app.Commit)
 	}
-	a.db.mtx.RUnlock()
 
-	return merr.Err()
+	return g.Wait()
 }
 
 // appendable returns a copy of a slice of HeadBlocks that can still be appended to.
diff --git a/querier.go b/querier.go
index 3ee90dd6f..d4e71e821 100644
--- a/querier.go
+++ b/querier.go
@@ -258,6 +258,9 @@ func (nopSeriesSet) Next() bool { return false }
 func (nopSeriesSet) At() Series { return nil }
 func (nopSeriesSet) Err() error { return nil }
 
+// mergedSeriesSet takes two series sets as a single series set. The input series sets
+// must be sorted and sequential in time, i.e. if they have the same label set,
+// the datapoints of a must be before the datapoints of b.
 type mergedSeriesSet struct {
 	a, b SeriesSet
 
@@ -307,11 +310,9 @@ func (s *mergedSeriesSet) Next() bool {
 	if d > 0 {
 		s.cur = s.b.At()
 		s.bdone = !s.b.Next()
-
 	} else if d < 0 {
 		s.cur = s.a.At()
 		s.adone = !s.a.Next()
-
 	} else {
 		s.cur = &chainedSeries{series: []Series{s.a.At(), s.b.At()}}
 		s.adone = !s.a.Next()