From 5a754bc043b8e8c2d57cf906386734a73761e2a2 Mon Sep 17 00:00:00 2001
From: Bryan Boreham <bjboreham@gmail.com>
Date: Tue, 21 Sep 2021 13:07:36 +0100
Subject: [PATCH] Short-circuit vector binary ops (#9362)

In degenerate cases we can save the effort of building a map.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
---
 promql/bench_test.go |  3 +++
 promql/engine.go     | 16 ++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/promql/bench_test.go b/promql/bench_test.go
index a1501a3cb..eb020d2f1 100644
--- a/promql/bench_test.go
+++ b/promql/bench_test.go
@@ -130,6 +130,9 @@ func BenchmarkRangeQuery(b *testing.B) {
 		{
 			expr: "a_X unless b_X{l=~'.*[0-4]$'}",
 		},
+		{
+			expr: "a_X and b_X{l='notfound'}",
+		},
 		// Simple functions.
 		{
 			expr: "abs(a_X)",
diff --git a/promql/engine.go b/promql/engine.go
index 8379bf48e..3b1d35757 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -1767,6 +1767,9 @@ func (ev *evaluator) VectorAnd(lhs, rhs Vector, matching *parser.VectorMatching,
 	if matching.Card != parser.CardManyToMany {
 		panic("set operations must only use many-to-many matching")
 	}
+	if len(lhs) == 0 || len(rhs) == 0 {
+		return nil // Short-circuit: AND with nothing is nothing.
+	}
 
 	// The set of signatures for the right-hand side Vector.
 	rightSigs := map[string]struct{}{}
@@ -1788,6 +1791,11 @@ func (ev *evaluator) VectorOr(lhs, rhs Vector, matching *parser.VectorMatching,
 	if matching.Card != parser.CardManyToMany {
 		panic("set operations must only use many-to-many matching")
 	}
+	if len(lhs) == 0 { // Short-circuit.
+		return rhs
+	} else if len(rhs) == 0 {
+		return lhs
+	}
 
 	leftSigs := map[string]struct{}{}
 	// Add everything from the left-hand-side Vector.
@@ -1808,6 +1816,11 @@ func (ev *evaluator) VectorUnless(lhs, rhs Vector, matching *parser.VectorMatchi
 	if matching.Card != parser.CardManyToMany {
 		panic("set operations must only use many-to-many matching")
 	}
+	// Short-circuit: empty rhs means we will return everything in lhs;
+	// empty lhs means we will return empty - don't need to build a map.
+	if len(lhs) == 0 || len(rhs) == 0 {
+		return lhs
+	}
 
 	rightSigs := map[string]struct{}{}
 	for _, sh := range rhsh {
@@ -1827,6 +1840,9 @@ func (ev *evaluator) VectorBinop(op parser.ItemType, lhs, rhs Vector, matching *
 	if matching.Card == parser.CardManyToMany {
 		panic("many-to-many only allowed for set operators")
 	}
+	if len(lhs) == 0 || len(rhs) == 0 {
+		return nil // Short-circuit: nothing is going to match.
+	}
 
 	// The control flow below handles one-to-one or many-to-one matching.
 	// For one-to-many, swap sidedness and account for the swap when calculating