From 4076277cf0402f381730fed9a2b76b7d4c7cb94a Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@victoriametrics.com>
Date: Fri, 2 Sep 2022 19:46:25 +0300
Subject: [PATCH] app/vmselect/promql: evaluate `union()` args in parallel in
 order to increase query performance

Note that the parallel execution of `union()` args may take more memory and CPU time
than the sequential execution if args contain heavy queries, which may load all the available CPU,
disk and memory resources and vmselect and vmstorage levels.
---
 app/vmselect/promql/eval.go | 43 ++++++++++++++++++++++++++++++++++---
 docs/CHANGELOG.md           |  2 ++
 2 files changed, 42 insertions(+), 3 deletions(-)

diff --git a/app/vmselect/promql/eval.go b/app/vmselect/promql/eval.go
index d59770538..e0c15c7f2 100644
--- a/app/vmselect/promql/eval.go
+++ b/app/vmselect/promql/eval.go
@@ -301,7 +301,14 @@ func evalTransformFunc(qt *querytracer.Tracer, ec *EvalConfig, fe *metricsql.Fun
 			Err: fmt.Errorf(`unknown func %q`, fe.Name),
 		}
 	}
-	args, err := evalExprs(qt, ec, fe.Args)
+	var args [][]*timeseries
+	var err error
+	switch fe.Name {
+	case "", "union":
+		args, err = evalExprsInParallel(qt, ec, fe.Args)
+	default:
+		args, err = evalExprsSequentially(qt, ec, fe.Args)
+	}
 	if err != nil {
 		return nil, err
 	}
@@ -337,7 +344,7 @@ func evalAggrFunc(qt *querytracer.Tracer, ec *EvalConfig, ae *metricsql.AggrFunc
 			return evalRollupFunc(qt, ec, fe.Name, rf, ae, re, iafc)
 		}
 	}
-	args, err := evalExprs(qt, ec, ae.Args)
+	args, err := evalExprsInParallel(qt, ec, ae.Args)
 	if err != nil {
 		return nil, err
 	}
@@ -616,7 +623,7 @@ func tryGetArgRollupFuncWithMetricExpr(ae *metricsql.AggrFuncExpr) (*metricsql.F
 	return nil, nil
 }
 
-func evalExprs(qt *querytracer.Tracer, ec *EvalConfig, es []metricsql.Expr) ([][]*timeseries, error) {
+func evalExprsSequentially(qt *querytracer.Tracer, ec *EvalConfig, es []metricsql.Expr) ([][]*timeseries, error) {
 	var rvs [][]*timeseries
 	for _, e := range es {
 		rv, err := evalExpr(qt, ec, e)
@@ -628,6 +635,36 @@ func evalExprs(qt *querytracer.Tracer, ec *EvalConfig, es []metricsql.Expr) ([][
 	return rvs, nil
 }
 
+func evalExprsInParallel(qt *querytracer.Tracer, ec *EvalConfig, es []metricsql.Expr) ([][]*timeseries, error) {
+	if len(es) < 2 {
+		return evalExprsSequentially(qt, ec, es)
+	}
+	rvs := make([][]*timeseries, len(es))
+	errs := make([]error, len(es))
+	var wg sync.WaitGroup
+	for i, e := range es {
+		qt.Printf("eval function args in parallel")
+		wg.Add(1)
+		qtChild := qt.NewChild("eval arg %d", i)
+		go func(e metricsql.Expr, i int) {
+			defer func() {
+				qtChild.Done()
+				wg.Done()
+			}()
+			rv, err := evalExpr(qtChild, ec, e)
+			rvs[i] = rv
+			errs[i] = err
+		}(e, i)
+	}
+	wg.Wait()
+	for _, err := range errs {
+		if err != nil {
+			return nil, err
+		}
+	}
+	return rvs, nil
+}
+
 func evalRollupFuncArgs(qt *querytracer.Tracer, ec *EvalConfig, fe *metricsql.FuncExpr) ([]interface{}, *metricsql.RollupExpr, error) {
 	var re *metricsql.RollupExpr
 	rollupArgIdx := metricsql.GetRollupArgIdx(fe)
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index eb65f38d3..914c3dffb 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -15,6 +15,8 @@ The following tip changes can be tested by building VictoriaMetrics components f
 
 ## tip
 
+* FEATURE: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): evaluate `q1`, ..., `qN` in parallel when calculating `union(q1, .., qN)`. Previously [union](https://docs.victoriametrics.com/MetricsQL.html#union) args were evaluated sequentially. This could result in lower than expected performance.
+
 ## [v1.81.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.81.0)
 
 Released at 31-08-2022