app/vmselect: add round_digits query arg to /api/v1/query and /api/v1/query_range handlers for limiting the number of decimal digits after the point

app/vmselect/prometheus/prometheus.go

@@ -1089,6 +1089,7 @@ func QueryHandler(startTime time.Time, at *auth.Token, w http.ResponseWriter, r
 		QuotedRemoteAddr:   httpserver.GetQuotedRemoteAddr(r),
 		Deadline:           deadline,
 		LookbackDelta:      lookbackDelta,
+		RoundDigits:        getRoundDigits(r),
 		EnforcedTagFilters: etf,
 
 		DenyPartialResponse: searchutils.GetDenyPartialResponse(r),
@@ -1196,6 +1197,7 @@ func queryRangeHandler(startTime time.Time, at *auth.Token, w http.ResponseWrite
 		Deadline:           deadline,
 		MayCache:           mayCache,
 		LookbackDelta:      lookbackDelta,
+		RoundDigits:        getRoundDigits(r),
 		EnforcedTagFilters: etf,
 
 		DenyPartialResponse: searchutils.GetDenyPartialResponse(r),
@@ -1374,6 +1376,18 @@ func getMatchesFromRequest(r *http.Request) []string {
 	return matches
 }
 
+func getRoundDigits(r *http.Request) int {
+	s := r.FormValue("round_digits")
+	if len(s) == 0 {
+		return 100
+	}
+	n, err := strconv.Atoi(s)
+	if err != nil {
+		return 100
+	}
+	return n
+}
+
 func getLatencyOffsetMilliseconds() int64 {
 	d := latencyOffset.Milliseconds()
 	if d <= 1000 {

app/vmselect/promql/eval.go

@@ -100,9 +100,13 @@ type EvalConfig struct {
 	// LookbackDelta is analog to `-query.lookback-delta` from Prometheus.
 	LookbackDelta int64
 
+	// How many decimal digits after the point to leave in response.
+	RoundDigits int
+
 	// EnforcedTagFilters used for apply additional label filters to query.
 	EnforcedTagFilters []storage.TagFilter
 
+	// Whether to deny partial response.
 	DenyPartialResponse bool
 
 	// IsPartialResponse is set during query execution and can be used by Exec caller after query execution.
@@ -122,6 +126,7 @@ func newEvalConfig(src *EvalConfig) *EvalConfig {
 	ec.Deadline = src.Deadline
 	ec.MayCache = src.MayCache
 	ec.LookbackDelta = src.LookbackDelta
+	ec.RoundDigits = src.RoundDigits
 	ec.EnforcedTagFilters = src.EnforcedTagFilters
 	ec.DenyPartialResponse = src.DenyPartialResponse
 	ec.IsPartialResponse = src.IsPartialResponse

app/vmselect/promql/exec.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmselect/netstorage"
 	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmselect/querystats"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/decimal"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 	"github.com/VictoriaMetrics/metrics"
 	"github.com/VictoriaMetrics/metricsql"
@@ -73,6 +74,14 @@ func Exec(ec *EvalConfig, q string, isFirstPointOnly bool) ([]netstorage.Result,
 	if err != nil {
 		return nil, err
 	}
+	if n := ec.RoundDigits; n < 100 {
+		for i := range result {
+			values := result[i].Values
+			for j, v := range values {
+				values[j] = decimal.RoundToDecimalDigits(v, n)
+			}
+		}
+	}
 	return result, err
 }
 

docs/CHANGELOG.md

@@ -12,6 +12,7 @@
 * FATURE: vmagent: accept `scrape_offset` option at `scrape_config`. This option may be useful when scrapes must start at the specified offset of every scrape interval. See [these docs](https://victoriametrics.github.io/vmagent.html#troubleshooting) for details.
 * FEATURE: vmagent: support `proxy_tls_config`, `proxy_basic_auth`, `proxy_bearer_token` and `proxy_bearer_token_file` options at `scrape_config` section for configuring proxies specified via `proxy_url`. See [these docs](https://victoriametrics.github.io/vmagent.html#scraping-targets-via-a-proxy).
 * FEATURE: vmauth: allow using regexp paths in `url_map`. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1112) for details.
+* FEATURE: accept `round_digits` query arg at `/api/v1/query` and `/api/v1/query_range` handlers. This option can be set at Prometheus datasource in Grafana for limiting the number of digits after the decimal point in response values.
 
 * BUGFIX: vmagent: prevent from high CPU usage bug during failing scrapes with small `scrape_timeout` (less than a few seconds).
 * BUGFIX: vmagent: reduce memory usage when Kubernetes service discovery is used in big number of distinct scrape config jobs by sharing Kubernetes object cache. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1113

docs/Single-server-VictoriaMetrics.md

@@ -562,14 +562,17 @@ in front of VictoriaMetrics. [Contact us](mailto:sales@victoriametrics.com) if y
 VictoriaMetrics accepts relative times in `time`, `start` and `end` query args additionally to unix timestamps and [RFC3339](https://www.ietf.org/rfc/rfc3339.txt).
 For example, the following query would return data for the last 30 minutes: `/api/v1/query_range?start=-30m&query=...`.
 
+VictoriaMetrics accepts `round_digits` query arg for `/api/v1/query` and `/api/v1/query_range` handlers. It can be used for rounding response values to the given number of digits after the decimal point. For example, `/api/v1/query?query=avg_over_time(temperature[1h])&round_digits=2` would round response values to up to two digits after the decimal point.
+
 By default, VictoriaMetrics returns time series for the last 5 minutes from `/api/v1/series`, while the Prometheus API defaults to all time.  Use `start` and `end` to select a different time range.
 
 VictoriaMetrics accepts additional args for `/api/v1/labels` and `/api/v1/label/.../values` handlers.
-See [this feature request](https://github.com/prometheus/prometheus/issues/6178) for details:
 
 * Any number [time series selectors](https://prometheus.io/docs/prometheus/latest/querying/basics/#time-series-selectors) via `match[]` query arg.
 * Optional `start` and `end` query args for limiting the time range for the selected labels or label values.
 
+See [this feature request](https://github.com/prometheus/prometheus/issues/6178) for details.
+
 Additionally VictoriaMetrics provides the following handlers:
 
 * `/api/v1/series/count` - returns the total number of time series in the database. Some notes: