From eb8c9759fc9b6cb99e9e8039b0f9a4c74d7f8656 Mon Sep 17 00:00:00 2001
From: Fabian Reinartz <fab.reinartz@gmail.com>
Date: Tue, 13 Jun 2017 08:25:13 +0200
Subject: [PATCH] Properly balance k-way operations

---
 postings.go | 18 ++++++++----------
 querier.go  | 44 ++++++++++++++++++++++++--------------------
 2 files changed, 32 insertions(+), 30 deletions(-)

diff --git a/postings.go b/postings.go
index f2f452ac1..f2f1eb5b8 100644
--- a/postings.go
+++ b/postings.go
@@ -78,12 +78,11 @@ func Intersect(its ...Postings) Postings {
 	if len(its) == 0 {
 		return emptyPostings
 	}
-	a := its[0]
-
-	for _, b := range its[1:] {
-		a = newIntersectPostings(a, b)
+	if len(its) == 1 {
+		return its[0]
 	}
-	return a
+	l := len(its) / 2
+	return newIntersectPostings(Intersect(its[:l]...), Intersect(its[l:]...))
 }
 
 type intersectPostings struct {
@@ -145,12 +144,11 @@ func Merge(its ...Postings) Postings {
 	if len(its) == 0 {
 		return nil
 	}
-	a := its[0]
-
-	for _, b := range its[1:] {
-		a = newMergedPostings(a, b)
+	if len(its) == 1 {
+		return its[0]
 	}
-	return a
+	l := len(its) / 2
+	return newMergedPostings(Merge(its[:l]...), Merge(its[l:]...))
 }
 
 type mergedPostings struct {
diff --git a/querier.go b/querier.go
index 601fc7440..751de0cb0 100644
--- a/querier.go
+++ b/querier.go
@@ -75,22 +75,26 @@ func (s *DB) Querier(mint, maxt int64) Querier {
 }
 
 func (q *querier) LabelValues(n string) ([]string, error) {
-	if len(q.blocks) == 0 {
+	return q.lvals(q.blocks, n)
+}
+
+func (q *querier) lvals(qs []Querier, n string) ([]string, error) {
+	if len(qs) == 0 {
 		return nil, nil
 	}
-	res, err := q.blocks[0].LabelValues(n)
+	if len(qs) == 1 {
+		return qs[0].LabelValues(n)
+	}
+	l := len(qs) / 2
+	s1, err := q.lvals(qs[:l], n)
 	if err != nil {
 		return nil, err
 	}
-	for _, bq := range q.blocks[1:] {
-		pr, err := bq.LabelValues(n)
-		if err != nil {
-			return nil, err
-		}
-		// Merge new values into deduplicated result.
-		res = mergeStrings(res, pr)
+	s2, err := q.lvals(qs[l:], n)
+	if err != nil {
+		return nil, err
 	}
-	return res, nil
+	return mergeStrings(s1, s2), nil
 }
 
 func (q *querier) LabelValuesFor(string, labels.Label) ([]string, error) {
@@ -98,19 +102,19 @@ func (q *querier) LabelValuesFor(string, labels.Label) ([]string, error) {
 }
 
 func (q *querier) Select(ms ...labels.Matcher) SeriesSet {
-	// Sets from different blocks have no time overlap. The reference numbers
-	// they emit point to series sorted in lexicographic order.
-	// We can fully connect partial series by simply comparing with the previous
-	// label set.
-	if len(q.blocks) == 0 {
+	return q.sel(q.blocks, ms)
+
+}
+
+func (q *querier) sel(qs []Querier, ms []labels.Matcher) SeriesSet {
+	if len(qs) == 0 {
 		return nopSeriesSet{}
 	}
-	r := q.blocks[0].Select(ms...)
-
-	for _, s := range q.blocks[1:] {
-		r = newMergedSeriesSet(r, s.Select(ms...))
+	if len(qs) == 1 {
+		return qs[0].Select(ms...)
 	}
-	return r
+	l := len(qs) / 2
+	return newMergedSeriesSet(q.sel(qs[:l], ms), q.sel(qs[l:], ms))
 }
 
 func (q *querier) Close() error {