lib/storage: improve performance for /api/v1/label/labelName/values when match[] contains only a single filter on labelName

This speeds up auto-suggestion for metric names in VMUI and Grafana, which use the following query in this case:

  /api/v1/label/__name__/values?match[]={__name__=~"*.some_value.*"}

When the user types `some_value` in the query input field.

docs/CHANGELOG.md

@@ -35,6 +35,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth/): support automatic discovering and load balancing for TCP addresses behind DNS SRV addresses. These addresses can be put inside `url_prefix` urls in the form `http://srv+addr/path`, where the `addr` is the [DNS SRV](https://en.wikipedia.org/wiki/SRV_record) address, which is automatically resolved to hostnames with TCP ports. See [these docs](https://docs.victoriametrics.com/vmauth/#srv-urls) for details.
 * FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth/): support specifying client TLS certificates and TLS ServerName for requests to HTTPS backends. See [these docs](https://docs.victoriametrics.com/vmauth/#backend-tls-setup).
 * FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth/): support regex matching when routing incoming requests based on HTTP [query args](https://en.wikipedia.org/wiki/Query_string) via `src_query_args` option at `url_map`. See [these docs](https://docs.victoriametrics.com/vmauth/#generic-http-proxy-for-different-backends) and [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6070).
+* FEATURE: [vmui](https://docs.victoriametrics.com/#vmui): optimize auto-suggestion performance for metric names when the database contains big number of unique time series.
 * FEATURE: [vmui](https://docs.victoriametrics.com/#vmui): in the Select component, user-entered values are now preserved on blur if they match options in the list.
 
 * BUGFIX: [vmalert](https://docs.victoriametrics.com/vmalert.html): supported any status codes from the range 200-299 from alertmanager. Previously, only 200 status code considered a successful action. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6110).

lib/storage/storage.go

@@ -1282,7 +1282,42 @@ func (s *Storage) SearchLabelNamesWithFiltersOnTimeRange(qt *querytracer.Tracer,
 func (s *Storage) SearchLabelValuesWithFiltersOnTimeRange(qt *querytracer.Tracer, labelName string, tfss []*TagFilters,
 	tr TimeRange, maxLabelValues, maxMetrics int, deadline uint64,
 ) ([]string, error) {
-	return s.idb().SearchLabelValuesWithFiltersOnTimeRange(qt, labelName, tfss, tr, maxLabelValues, maxMetrics, deadline)
+	idb := s.idb()
+
+	key := labelName
+	if key == "__name__" {
+		key = ""
+	}
+	if len(tfss) == 1 && len(tfss[0].tfs) == 1 && string(tfss[0].tfs[0].key) == key {
+		// tfss contains only a single filter on labelName. It is faster searching for label values
+		// without any filters and then later applying the filter to the found label values.
+		lvs, err := idb.SearchLabelValuesWithFiltersOnTimeRange(qt, labelName, nil, tr, maxLabelValues, maxMetrics, deadline)
+		if err != nil {
+			return nil, err
+		}
+		lvs = filterLabelValues(lvs, &tfss[0].tfs[0], key)
+		return lvs, nil
+	}
+
+	return idb.SearchLabelValuesWithFiltersOnTimeRange(qt, labelName, tfss, tr, maxLabelValues, maxMetrics, deadline)
+}
+
+func filterLabelValues(lvs []string, tf *tagFilter, key string) []string {
+	var b []byte
+	result := lvs[:0]
+	for _, lv := range lvs {
+		b = marshalCommonPrefix(b[:0], nsPrefixTagToMetricIDs)
+		b = marshalTagValue(b, bytesutil.ToUnsafeBytes(key))
+		b = marshalTagValue(b, bytesutil.ToUnsafeBytes(lv))
+		ok, err := tf.match(b)
+		if err != nil {
+			logger.Panicf("BUG: cannot match label %q=%q with tagFilter %s: %w", key, lv, tf.String(), err)
+		}
+		if ok {
+			result = append(result, lv)
+		}
+	}
+	return result
 }
 
 // SearchTagValueSuffixes returns all the tag value suffixes for the given tagKey and tagValuePrefix on the given tr.