Implement histogram_quantile()

Signed-off-by: Julius Volz <[email protected]>

promflux/call.go

@@ -426,6 +426,22 @@ func (t *transpiler) transpileCall(c *promql.Call) (ast.Expression, error) {
 				"fn": setConstValueFn(args[0]),
 			}),
 		), nil
+	case "histogram_quantile":
+		if yieldsTable(c.Args[0]) {
+			return nil, fmt.Errorf("non-const scalar expressions not supported yet")
+		}
+
+		return buildPipeline(
+			args[1],
+			call("group", map[string]ast.Expression{
+				"columns": columnList("_time", "_value", "le"),
+				"mode":    &ast.StringLiteral{Value: "except"},
+			}),
+			call("promql.promHistogramQuantile", map[string]ast.Expression{
+				"quantile": args[0],
+			}),
+			dropMeasurementCall,
+		), nil
 	default:
 		return nil, fmt.Errorf("PromQL function %q is not supported yet", c.Func.Name)
 	}

promflux/query_test.go

@@ -354,6 +354,21 @@ var queries = []struct {
 	{
 		query: `vector(time())`,
 	},
+	{
+		query:       `histogram_quantile({{.quantile}}, rate(demo_api_request_duration_seconds_bucket[1m]))`,
+		variantArgs: []string{"quantile"},
+	},
+	{
+		query: `histogram_quantile(0.9, nonexistent_metric)`,
+	},
+	{
+		// Missing "le" label.
+		query: `histogram_quantile(0.9, demo_cpu_usage_seconds_total)`,
+	},
+	{
+		// Missing "le" label only in some series of the same grouping.
+		query: `histogram_quantile(0.9, {__name__=~"demo_api_request_duration_seconds_.+"})`,
+	},
 
 	// Subqueries. Comparisons are skipped since the implementation cannot guarantee completely identical results.
 	{