From 092c9b39a8f2bdc10497ad134906ff5ccec0eabc Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@gmail.com>
Date: Fri, 12 Jul 2019 19:56:23 +0300
Subject: [PATCH] app/vmselect/promql: remove empty time series after applying
 filters like `q > 0`

This should reduce CPU and RAM usage for queries over high number of time series.
---
 app/vmselect/promql/binary_op.go |  3 +++
 app/vmselect/promql/exec.go      | 13 +++++++++----
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/app/vmselect/promql/binary_op.go b/app/vmselect/promql/binary_op.go
index f8ff4c60a..7bde1cab5 100644
--- a/app/vmselect/promql/binary_op.go
+++ b/app/vmselect/promql/binary_op.go
@@ -260,6 +260,9 @@ func newBinaryOpFunc(bf func(left, right float64, isBool bool) float64) binaryOp
 				dstValues[j] = bf(a, b, isBool)
 			}
 		}
+		// Optimization: remove time series containing only NaNs.
+		// This is quite common after applying filters like `q > 0`.
+		dst = removeNaNs(dst)
 		return dst, nil
 	}
 }
diff --git a/app/vmselect/promql/exec.go b/app/vmselect/promql/exec.go
index 42ffbd6ae..c7b0e2f39 100644
--- a/app/vmselect/promql/exec.go
+++ b/app/vmselect/promql/exec.go
@@ -131,18 +131,23 @@ func timeseriesToResult(tss []*timeseries, maySort bool) ([]netstorage.Result, e
 func removeNaNs(tss []*timeseries) []*timeseries {
 	rvs := tss[:0]
 	for _, ts := range tss {
-		nans := 0
+		allNans := true
 		for _, v := range ts.Values {
-			if math.IsNaN(v) {
-				nans++
+			if !math.IsNaN(v) {
+				allNans = false
+				break
 			}
 		}
-		if nans == len(ts.Values) {
+		if allNans {
 			// Skip timeseries with all NaNs.
 			continue
 		}
 		rvs = append(rvs, ts)
 	}
+	for i := len(rvs); i < len(tss); i++ {
+		// Zero unused time series, so GC could reclaim them.
+		tss[i] = nil
+	}
 	return rvs
 }