From 0c05f95e929f1b62f500c9ed4a806a768acc34aa Mon Sep 17 00:00:00 2001
From: Bryan Boreham <bjboreham@gmail.com>
Date: Sat, 26 Nov 2022 14:56:22 +0000
Subject: [PATCH] tsdb: use smaller allocation in blockBaseSeriesSet

This reduces garbage, hence goes faster, when a short time range is
required compared to the amount of chunks in the block. For example
recording rules and alerts often look only at the last few minutes.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
---
 tsdb/querier.go | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/tsdb/querier.go b/tsdb/querier.go
index 70b384e02..cf6b3c9dc 100644
--- a/tsdb/querier.go
+++ b/tsdb/querier.go
@@ -471,7 +471,14 @@ func (b *blockBaseSeriesSet) Next() bool {
 		var trimFront, trimBack bool
 
 		// Copy chunks as iterables are reusable.
-		chks := make([]chunks.Meta, 0, len(b.bufChks))
+		// Count those in range to size allocation (roughly - ignoring tombstones).
+		nChks := 0
+		for _, chk := range b.bufChks {
+			if !(chk.MaxTime < b.mint || chk.MinTime > b.maxt) {
+				nChks++
+			}
+		}
+		chks := make([]chunks.Meta, 0, nChks)
 
 		// Prefilter chunks and pick those which are not entirely deleted or totally outside of the requested range.
 		for _, chk := range b.bufChks {