diff --git a/lib/mergeset/encoding.go b/lib/mergeset/encoding.go
index a347084fa..94eaa1415 100644
--- a/lib/mergeset/encoding.go
+++ b/lib/mergeset/encoding.go
@@ -11,10 +11,20 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 )
 
+type byteSliceSorter [][]byte
+
+func (s byteSliceSorter) Len() int { return len(s) }
+func (s byteSliceSorter) Less(i, j int) bool {
+	return string(s[i]) < string(s[j])
+}
+func (s byteSliceSorter) Swap(i, j int) {
+	s[i], s[j] = s[j], s[i]
+}
+
 type inmemoryBlock struct {
 	commonPrefix []byte
 	data         []byte
-	items        [][]byte
+	items        byteSliceSorter
 }
 
 func (ib *inmemoryBlock) Reset() {
@@ -77,12 +87,9 @@ func (ib *inmemoryBlock) Add(x []byte) bool {
 // It must fit CPU cache size, i.e. 64KB for the current CPUs.
 const maxInmemoryBlockSize = 64 * 1024
 
-func (ib *inmemoryBlock) itemsLess(i, j int) bool {
-	return string(ib.items[i]) < string(ib.items[j])
-}
-
 func (ib *inmemoryBlock) sort() {
-	sort.Slice(ib.items, ib.itemsLess)
+	// Use sort.Sort instead of sort.Slice in order to eliminate memory allocation.
+	sort.Sort(&ib.items)
 	bb := bbPool.Get()
 	b := bytesutil.Resize(bb.B, len(ib.data))
 	b = b[:0]
@@ -120,7 +127,8 @@ func checkMarshalType(mt marshalType) error {
 }
 
 func (ib *inmemoryBlock) isSorted() bool {
-	return sort.SliceIsSorted(ib.items, ib.itemsLess)
+	// Use sort.IsSorted instead of sort.SliceIsSorted in order to eliminate memory allocation.
+	return sort.IsSorted(&ib.items)
 }
 
 // MarshalUnsortedData marshals unsorted items from ib to sb.
@@ -138,6 +146,8 @@ func (ib *inmemoryBlock) MarshalUnsortedData(sb *storageBlock, firstItemDst, com
 	return ib.marshalData(sb, firstItemDst, commonPrefixDst, compressLevel)
 }
 
+var isInTest bool
+
 // MarshalUnsortedData marshals sorted items from ib to sb.
 //
 // It also:
@@ -146,9 +156,9 @@ func (ib *inmemoryBlock) MarshalUnsortedData(sb *storageBlock, firstItemDst, com
 // - returns the number of items encoded including the first item.
 // - returns the marshal type used for the encoding.
 func (ib *inmemoryBlock) MarshalSortedData(sb *storageBlock, firstItemDst, commonPrefixDst []byte, compressLevel int) ([]byte, []byte, uint32, marshalType) {
-	// if !ib.isSorted() {
-	//	logger.Panicf("BUG: %d items must be sorted; items:\n%s", len(ib.items), ib.debugItemsString())
-	// }
+	if isInTest && !ib.isSorted() {
+		logger.Panicf("BUG: %d items must be sorted; items:\n%s", len(ib.items), ib.debugItemsString())
+	}
 	ib.updateCommonPrefix()
 	return ib.marshalData(sb, firstItemDst, commonPrefixDst, compressLevel)
 }
diff --git a/lib/mergeset/table_search_test.go b/lib/mergeset/table_search_test.go
index cfb721bb9..806e2a887 100644
--- a/lib/mergeset/table_search_test.go
+++ b/lib/mergeset/table_search_test.go
@@ -11,6 +11,7 @@ import (
 )
 
 func TestMain(m *testing.M) {
+	isInTest = true
 	n := m.Run()
 	os.Exit(n)
 }