add comment about why we batch

influxql/engine.go

@@ -755,7 +755,9 @@ func MapStddev(itr Iterator, e *Emitter, tmax int64) {
 
 	for k, v := itr.Next(); k != 0; k, v = itr.Next() {
 		values = append(values, v.(float64))
-		// Emit in batches
+		// Emit in batches.
+		// unbounded emission of data can lead to excessive memory use
+		// or other potential performance problems.
 		if len(values) == emitBatchSize {
 			e.Emit(Key{tmax, itr.Tags()}, values)
 			values = []float64{}