[PERF] TSDB: Speed up intersectPostings.Next

Check if the next position is already a match, in which case we don't
have to call `Seek`.

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

tsdb/index/postings.go

@@ -641,15 +641,28 @@ func (it *intersectPostings) Seek(target storage.SeriesRef) bool {
 }
 
 func (it *intersectPostings) Next() bool {
-	target := it.current
+	// Move forward the first Postings and take its value as the target to match.
-	for _, p := range it.postings {
+	if !it.postings[0].Next() {
+		return false
+	}
+	target := it.postings[0].At()
+	allEqual := true
+	for _, p := range it.postings[1:] { // Now move forward all the other ones and check if they match.
 		if !p.Next() {
 			return false
 		}
-		if p.At() > target {
+		at := p.At()
-			target = p.At()
+		if at > target { // This one is past the target, so pick up a new target to Seek at the end.
+			target = at
+			allEqual = false
+		} else if at < target { // This one needs to Seek to the target, but carry on with other postings in case they have an even higher target.
+			allEqual = false
 		}
 	}
+	if allEqual {
+		it.current = target
+		return true
+	}
 	return it.Seek(target)
 }