From 2fb50b12cd3ad42b1f20a331d5a5e5402469c17f Mon Sep 17 00:00:00 2001
From: Bryan Boreham <bjboreham@gmail.com>
Date: Wed, 27 Aug 2025 17:11:08 +0100
Subject: [PATCH] [PERF] TSDB: Optimize appender creation on empty chunks
 (#16922)

Skip creating an iterator and walking all through any existing values,
when we can easily tell there are no existing values.

This is the normal case - the TSDB head creates an appender immediately
after creating every chunk.

Remove redundant handling of empty chunks.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
---
 tsdb/chunkenc/float_histogram.go | 8 +++-----
 tsdb/chunkenc/histogram.go       | 6 +++---
 tsdb/chunkenc/xor.go             | 6 +++---
 3 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/tsdb/chunkenc/float_histogram.go b/tsdb/chunkenc/float_histogram.go
index 05f5d5e252..bb37f44395 100644
--- a/tsdb/chunkenc/float_histogram.go
+++ b/tsdb/chunkenc/float_histogram.go
@@ -89,6 +89,9 @@ func (c *FloatHistogramChunk) Compact() {
 
 // Appender implements the Chunk interface.
 func (c *FloatHistogramChunk) Appender() (Appender, error) {
+	if len(c.b.stream) == 3 { // Avoid allocating an Iterator when chunk is empty.
+		return &FloatHistogramAppender{b: &c.b, t: math.MinInt64, sum: xorValue{leading: 0xff}, cnt: xorValue{leading: 0xff}, zCnt: xorValue{leading: 0xff}}, nil
+	}
 	it := c.iterator(nil)
 
 	// To get an appender, we must know the state it would have if we had
@@ -133,11 +136,6 @@ func (c *FloatHistogramChunk) Appender() (Appender, error) {
 		nBuckets:     nBuckets,
 		sum:          it.sum,
 	}
-	if it.numTotal == 0 {
-		a.sum.leading = 0xff
-		a.cnt.leading = 0xff
-		a.zCnt.leading = 0xff
-	}
 	return a, nil
 }
 
diff --git a/tsdb/chunkenc/histogram.go b/tsdb/chunkenc/histogram.go
index 1b511c1f7b..e818ca4d91 100644
--- a/tsdb/chunkenc/histogram.go
+++ b/tsdb/chunkenc/histogram.go
@@ -100,6 +100,9 @@ func (c *HistogramChunk) Compact() {
 
 // Appender implements the Chunk interface.
 func (c *HistogramChunk) Appender() (Appender, error) {
+	if len(c.b.stream) == 3 { // Avoid allocating an Iterator when chunk is empty.
+		return &HistogramAppender{b: &c.b, t: math.MinInt64, leading: 0xff}, nil
+	}
 	it := c.iterator(nil)
 
 	// To get an appender, we must know the state it would have if we had
@@ -134,9 +137,6 @@ func (c *HistogramChunk) Appender() (Appender, error) {
 		leading:  it.leading,
 		trailing: it.trailing,
 	}
-	if it.numTotal == 0 {
-		a.leading = 0xff
-	}
 	return a, nil
 }
 
diff --git a/tsdb/chunkenc/xor.go b/tsdb/chunkenc/xor.go
index 5a37ebfea9..aac0f546d7 100644
--- a/tsdb/chunkenc/xor.go
+++ b/tsdb/chunkenc/xor.go
@@ -98,6 +98,9 @@ func (c *XORChunk) Compact() {
 // It is not valid to call Appender() multiple times concurrently or to use multiple
 // Appenders on the same chunk.
 func (c *XORChunk) Appender() (Appender, error) {
+	if len(c.b.stream) == 2 { // Avoid allocating an Iterator when chunk is empty.
+		return &xorAppender{b: &c.b, t: math.MinInt64, leading: 0xff}, nil
+	}
 	it := c.iterator(nil)
 
 	// To get an appender we must know the state it would have if we had
@@ -117,9 +120,6 @@ func (c *XORChunk) Appender() (Appender, error) {
 		leading:  it.leading,
 		trailing: it.trailing,
 	}
-	if it.numTotal == 0 {
-		a.leading = 0xff
-	}
 	return a, nil
 }