[ENHANCEMENT] Storage: Short-circuit merge of single querier with no-op queriers

Filter before checking whether there is only one.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

storage/merge.go

@@ -45,8 +45,11 @@ type mergeGenericQuerier struct {
 //
 // In case of overlaps between the data given by primaries' and secondaries' Selects, merge function will be used.
 func NewMergeQuerier(primaries, secondaries []Querier, mergeFn VerticalSeriesMergeFunc) Querier {
+	primaries = filterQueriers(primaries)
+	secondaries = filterQueriers(secondaries)
+
 	switch {
-	case len(primaries)+len(secondaries) == 0:
+	case len(primaries) == 0 && len(secondaries) == 0:
 		return noopQuerier{}
 	case len(primaries) == 1 && len(secondaries) == 0:
 		return primaries[0]
@@ -56,14 +59,10 @@ func NewMergeQuerier(primaries, secondaries []Querier, mergeFn VerticalSeriesMer
 
 	queriers := make([]genericQuerier, 0, len(primaries)+len(secondaries))
 	for _, q := range primaries {
-		if _, ok := q.(noopQuerier); !ok && q != nil {
-			queriers = append(queriers, newGenericQuerierFrom(q))
-		}
+		queriers = append(queriers, newGenericQuerierFrom(q))
 	}
 	for _, q := range secondaries {
-		if _, ok := q.(noopQuerier); !ok && q != nil {
-			queriers = append(queriers, newSecondaryQuerierFrom(q))
-		}
+		queriers = append(queriers, newSecondaryQuerierFrom(q))
 	}
 
 	concurrentSelect := false
@@ -77,12 +76,25 @@ func NewMergeQuerier(primaries, secondaries []Querier, mergeFn VerticalSeriesMer
 	}}
 }
 
+func filterQueriers(qs []Querier) []Querier {
+	ret := make([]Querier, 0, len(qs))
+	for _, q := range qs {
+		if _, ok := q.(noopQuerier); !ok && q != nil {
+			ret = append(ret, q)
+		}
+	}
+	return ret
+}
+
 // NewMergeChunkQuerier returns a new Chunk Querier that merges results of given primary and secondary chunk queriers.
 // See NewFanout commentary to learn more about primary vs secondary differences.
 //
 // In case of overlaps between the data given by primaries' and secondaries' Selects, merge function will be used.
 // TODO(bwplotka): Currently merge will compact overlapping chunks with bigger chunk, without limit. Split it: https://github.com/prometheus/tsdb/issues/670
 func NewMergeChunkQuerier(primaries, secondaries []ChunkQuerier, mergeFn VerticalChunkSeriesMergeFunc) ChunkQuerier {
+	primaries = filterChunkQueriers(primaries)
+	secondaries = filterChunkQueriers(secondaries)
+
 	switch {
 	case len(primaries) == 0 && len(secondaries) == 0:
 		return noopChunkQuerier{}
@@ -94,14 +106,10 @@ func NewMergeChunkQuerier(primaries, secondaries []ChunkQuerier, mergeFn Vertica
 
 	queriers := make([]genericQuerier, 0, len(primaries)+len(secondaries))
 	for _, q := range primaries {
-		if _, ok := q.(noopChunkQuerier); !ok && q != nil {
-			queriers = append(queriers, newGenericQuerierFromChunk(q))
-		}
+		queriers = append(queriers, newGenericQuerierFromChunk(q))
 	}
-	for _, querier := range secondaries {
-		if _, ok := querier.(noopChunkQuerier); !ok && querier != nil {
-			queriers = append(queriers, newSecondaryQuerierFromChunk(querier))
-		}
+	for _, q := range secondaries {
+		queriers = append(queriers, newSecondaryQuerierFromChunk(q))
 	}
 
 	concurrentSelect := false
@@ -115,6 +123,16 @@ func NewMergeChunkQuerier(primaries, secondaries []ChunkQuerier, mergeFn Vertica
 	}}
 }
 
+func filterChunkQueriers(qs []ChunkQuerier) []ChunkQuerier {
+	ret := make([]ChunkQuerier, 0, len(qs))
+	for _, q := range qs {
+		if _, ok := q.(noopChunkQuerier); !ok && q != nil {
+			ret = append(ret, q)
+		}
+	}
+	return ret
+}
+
 // Select returns a set of series that matches the given label matchers.
 func (q *mergeGenericQuerier) Select(ctx context.Context, sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) genericSeriesSet {
 	seriesSets := make([]genericSeriesSet, 0, len(q.queriers))