diff --git a/lib/storage/index_db.go b/lib/storage/index_db.go
index c3e2785b8..7ee94b157 100644
--- a/lib/storage/index_db.go
+++ b/lib/storage/index_db.go
@@ -146,7 +146,7 @@ func mustOpenIndexDB(path string, s *Storage, isReadOnly *uint32) *indexDB {
 
 	tb := mergeset.MustOpenTable(path, invalidateTagFiltersCache, mergeTagToMetricIDsRows, isReadOnly)
 
-	// Do not persist tagFiltersToMetricIDsCache in files, since it is very volatile.
+	// Do not persist tagFiltersToMetricIDsCache in files, since it is very volatile because of tagFiltersKeyGen.
 	mem := memory.Allowed()
 	tagFiltersCacheSize := getTagFiltersCacheSize()
 
@@ -320,10 +320,7 @@ func (db *indexDB) getMetricIDsFromTagFiltersCache(qt *querytracer.Tracer, key [
 		return nil, false
 	}
 	qt.Printf("found metricIDs with size: %d bytes", len(buf.B))
-	metricIDs, err := unmarshalMetricIDs(nil, buf.B)
-	if err != nil {
-		logger.Panicf("FATAL: cannot unmarshal metricIDs from tagFiltersToMetricIDsCache: %s", err)
-	}
+	metricIDs := mustUnmarshalMetricIDs(nil, buf.B)
 	qt.Printf("unmarshaled %d metricIDs", len(metricIDs))
 	return metricIDs, true
 }
@@ -374,6 +371,8 @@ func (db *indexDB) putMetricNameToCache(metricID uint64, metricName []byte) {
 }
 
 func marshalTagFiltersKey(dst []byte, tfss []*TagFilters, tr TimeRange, versioned bool) []byte {
+	// There is no need in versioning the tagFilters key, since the tagFiltersToMetricIDsCache
+	// isn't persisted to disk (it is very volatile because of tagFiltersKeyGen).
 	prefix := ^uint64(0)
 	if versioned {
 		prefix = atomic.LoadUint64(&tagFiltersKeyGen)
@@ -394,52 +393,60 @@ func marshalTagFiltersKey(dst []byte, tfss []*TagFilters, tr TimeRange, versione
 }
 
 func invalidateTagFiltersCache() {
-	// This function must be fast, since it is called each
-	// time new timeseries is added.
+	// This function must be fast, since it is called each time new timeseries is added.
 	atomic.AddUint64(&tagFiltersKeyGen, 1)
 }
 
 var tagFiltersKeyGen uint64
 
 func marshalMetricIDs(dst []byte, metricIDs []uint64) []byte {
-	dst = encoding.MarshalUint64(dst, uint64(len(metricIDs)))
-	if len(metricIDs) == 0 {
-		return dst
+	// Compress metricIDs, so they occupy less space in the cache.
+	//
+	// The srcBuf is a []byte cast of metricIDs.
+	var srcBuf []byte
+	if len(metricIDs) > 0 {
+		sh := (*reflect.SliceHeader)(unsafe.Pointer(&srcBuf))
+		sh.Data = uintptr(unsafe.Pointer(&metricIDs[0]))
+		sh.Cap = 8 * len(metricIDs)
+		sh.Len = 8 * len(metricIDs)
 	}
-	var buf []byte
-	sh := (*reflect.SliceHeader)(unsafe.Pointer(&buf))
-	sh.Data = uintptr(unsafe.Pointer(&metricIDs[0]))
-	sh.Cap = 8 * len(metricIDs)
-	sh.Len = sh.Cap
-	dst = append(dst, buf...)
+
+	dst = encoding.CompressZSTDLevel(dst, srcBuf, 1)
 	return dst
 }
 
-func unmarshalMetricIDs(dst []uint64, src []byte) ([]uint64, error) {
-	if len(src)%8 != 0 {
-		return dst, fmt.Errorf("cannot unmarshal metricIDs from buffer of %d bytes; the buffer length must divide by 8", len(src))
+func mustUnmarshalMetricIDs(dst []uint64, src []byte) []uint64 {
+	// Decompress src into dstBuf.
+	//
+	// dstBuf is a []byte cast of dst.
+	var dstBuf []byte
+	if len(dst) > 0 {
+		sh := (*reflect.SliceHeader)(unsafe.Pointer(&dstBuf))
+		sh.Data = uintptr(unsafe.Pointer(&dst[0]))
+		sh.Cap = 8 * cap(dst)
+		sh.Len = 8 * len(dst)
 	}
-	if len(src) < 8 {
-		return dst, fmt.Errorf("cannot unmarshal metricIDs len from buffer of %d bytes; need at least 8 bytes", len(src))
+	dstBufLen := len(dstBuf)
+	var err error
+	dstBuf, err = encoding.DecompressZSTD(dstBuf, src)
+	if err != nil {
+		logger.Panicf("FATAL: cannot decompress metricIDs: %s", err)
 	}
-	n := encoding.UnmarshalUint64(src)
-	if n > ((1<<64)-1)/8 {
-		return dst, fmt.Errorf("unexpectedly high metricIDs len: %d bytes; must be lower than %d bytes", n, uint64(((1<<64)-1)/8))
+	if len(dstBuf) == dstBufLen {
+		// Zero metricIDs
+		return dst
 	}
-	src = src[8:]
-	if n*8 != uint64(len(src)) {
-		return dst, fmt.Errorf("unexpected buffer length for unmarshaling metricIDs; got %d bytes; want %d bytes", n*8, len(src))
+	if (len(dstBuf)-dstBufLen)%8 != 0 {
+		logger.Panicf("FATAL: cannot unmarshal metricIDs from buffer of %d bytes; the buffer length must divide by 8", len(dstBuf)-dstBufLen)
 	}
-	if n == 0 {
-		return dst, nil
-	}
-	var metricIDs []uint64
-	sh := (*reflect.SliceHeader)(unsafe.Pointer(&metricIDs))
-	sh.Data = uintptr(unsafe.Pointer(&src[0]))
-	sh.Cap = sh.Len
-	sh.Len = len(src) / 8
-	dst = append(dst, metricIDs...)
-	return dst, nil
+
+	// Convert dstBuf back to dst
+	sh := (*reflect.SliceHeader)(unsafe.Pointer(&dst))
+	sh.Data = uintptr(unsafe.Pointer(&dstBuf[0]))
+	sh.Cap = cap(dstBuf) / 8
+	sh.Len = len(dstBuf) / 8
+
+	return dst
 }
 
 // getTSIDByMetricName fills the dst with TSID for the given metricName at the given date.
@@ -1582,6 +1589,9 @@ func (is *indexSearch) loadDeletedMetricIDs() (*uint64set.Set, error) {
 	return dmis, nil
 }
 
+// searchMetricIDs returns metricIDs for the given tfss and tr.
+//
+// The returned metricIDs are sorted.
 func (db *indexDB) searchMetricIDs(qt *querytracer.Tracer, tfss []*TagFilters, tr TimeRange, maxMetrics int, deadline uint64) ([]uint64, error) {
 	qt = qt.NewChild("search for matching metricIDs: filters=%s, timeRange=%s", tfss, &tr)
 	defer qt.Done()
@@ -2117,6 +2127,9 @@ func (is *indexSearch) searchMetricIDsWithFiltersOnDate(qt *querytracer.Tracer,
 	return metricIDs, nil
 }
 
+// searchMetricIDs returns metricIDs for the given tfss and tr.
+//
+// The returned metricIDs are sorted.
 func (is *indexSearch) searchMetricIDs(qt *querytracer.Tracer, tfss []*TagFilters, tr TimeRange, maxMetrics int) ([]uint64, error) {
 	ok, err := is.containsTimeRange(tr)
 	if err != nil {
diff --git a/lib/storage/index_db_test.go b/lib/storage/index_db_test.go
index 16f5f2dec..2e939ab1b 100644
--- a/lib/storage/index_db_test.go
+++ b/lib/storage/index_db_test.go
@@ -24,18 +24,44 @@ import (
 func TestMarshalUnmarshalMetricIDs(t *testing.T) {
 	f := func(metricIDs []uint64) {
 		t.Helper()
+
+		// Try marshaling and unmarshaling to an empty dst
 		data := marshalMetricIDs(nil, metricIDs)
-		result, err := unmarshalMetricIDs(nil, data)
-		if err != nil {
-			t.Fatalf("unexpected error: %s", err)
-		}
+		result := mustUnmarshalMetricIDs(nil, data)
 		if !reflect.DeepEqual(result, metricIDs) {
 			t.Fatalf("unexpected metricIDs after unmarshaling;\ngot\n%d\nwant\n%d", result, metricIDs)
 		}
+
+		// Try marshaling and unmarshaling to non-empty dst
+		dataPrefix := []byte("prefix")
+		data = marshalMetricIDs(dataPrefix, metricIDs)
+		if len(data) < len(dataPrefix) {
+			t.Fatalf("too short len(data)=%d; must be at least len(dataPrefix)=%d", len(data), len(dataPrefix))
+		}
+		if string(data[:len(dataPrefix)]) != string(dataPrefix) {
+			t.Fatalf("unexpected prefix; got %q; want %q", data[:len(dataPrefix)], dataPrefix)
+		}
+		data = data[len(dataPrefix):]
+
+		resultPrefix := []uint64{889432422, 89243, 9823}
+		result = mustUnmarshalMetricIDs(resultPrefix, data)
+		if len(result) < len(resultPrefix) {
+			t.Fatalf("too short result returned; len(result)=%d; must be at least len(resultPrefix)=%d", len(result), len(resultPrefix))
+		}
+		if !reflect.DeepEqual(result[:len(resultPrefix)], resultPrefix) {
+			t.Fatalf("unexpected result prefix; got %d; want %d", result[:len(resultPrefix)], resultPrefix)
+		}
+		result = result[len(resultPrefix):]
+		if (len(metricIDs) > 0 || len(result) > 0) && !reflect.DeepEqual(result, metricIDs) {
+			t.Fatalf("unexpected metricIDs after unmarshaling from prefix;\ngot\n%d\nwant\n%d", result, metricIDs)
+		}
 	}
+
 	f(nil)
+	f([]uint64{0})
 	f([]uint64{1})
 	f([]uint64{1234, 678932943, 843289893843})
+	f([]uint64{1, 2, 3, 4, 5, 6, 8989898, 823849234, 1<<64 - 1, 1<<32 - 1, 0})
 }
 
 func TestMergeSortedMetricIDs(t *testing.T) {