diff --git a/arrowexec/execution/expression.go b/arrowexec/execution/expression.go
index 58f55484..404052d7 100644
--- a/arrowexec/execution/expression.go
+++ b/arrowexec/execution/expression.go
@@ -18,6 +18,7 @@ type RecordVariable struct {
 }
 
 func (r *RecordVariable) Evaluate(ctx Context, record Record) (arrow.Array, error) {
+	// TODO: Retain array?
 	return record.Column(r.index), nil
 }
 
diff --git a/arrowexec/nodes/filter_test.go b/arrowexec/nodes/filter_test.go
index 1fa6dfcf..ee9fcdbf 100644
--- a/arrowexec/nodes/filter_test.go
+++ b/arrowexec/nodes/filter_test.go
@@ -68,9 +68,9 @@ func BenchmarkNaiveFilter(b *testing.B) {
 		Node: &GroupBy{
 			OutSchema:             schema,
 			Source:                node,
-			KeyExprs:              []int{0},
+			KeyColumns:            []int{0},
 			AggregateConstructors: []func(dt arrow.DataType) Aggregate{MakeCount},
-			AggregateExprs:        []int{1},
+			AggregateColumns:      []int{1},
 		},
 		Schema: schema,
 	}
@@ -132,9 +132,9 @@ func BenchmarkRebatchingFilter(b *testing.B) {
 		Node: &GroupBy{
 			OutSchema:             schema,
 			Source:                node,
-			KeyExprs:              []int{0},
+			KeyColumns:            []int{0},
 			AggregateConstructors: []func(dt arrow.DataType) Aggregate{MakeCount},
-			AggregateExprs:        []int{1},
+			AggregateColumns:      []int{1},
 		},
 		Schema: schema,
 	}
diff --git a/arrowexec/nodes/group_by.go b/arrowexec/nodes/group_by.go
index a1995e73..5391dca0 100644
--- a/arrowexec/nodes/group_by.go
+++ b/arrowexec/nodes/group_by.go
@@ -16,9 +16,11 @@ type GroupBy struct {
 	OutSchema *arrow.Schema
 	Source    execution.NodeWithMeta
 
-	KeyExprs              []int // For now, let's just use indices here.
+	// Both keys and aggregate columns have to be calculated by a preceding map.
+
+	KeyColumns            []int
 	AggregateConstructors []func(dt arrow.DataType) Aggregate
-	AggregateExprs        []int // For now, let's just use indices here.
+	AggregateColumns      []int
 }
 
 func (g *GroupBy) Run(ctx execution.Context, produce execution.ProduceFunc) error {
@@ -26,27 +28,27 @@ func (g *GroupBy) Run(ctx execution.Context, produce execution.ProduceFunc) erro
 	entryIndices := intintmap.New(16, 0.6)
 	aggregates := make([]Aggregate, len(g.AggregateConstructors))
 	for i := range aggregates {
-		aggregates[i] = g.AggregateConstructors[i](g.Source.Schema.Field(g.AggregateExprs[i]).Type)
+		aggregates[i] = g.AggregateConstructors[i](g.Source.Schema.Field(g.AggregateColumns[i]).Type)
 	}
-	key := make([]Key, len(g.KeyExprs))
+	key := make([]Key, len(g.KeyColumns))
 	for i := range key {
-		key[i] = MakeKey(g.Source.Schema.Field(g.KeyExprs[i]).Type)
+		key[i] = MakeKey(g.Source.Schema.Field(g.KeyColumns[i]).Type)
 	}
 
 	if err := g.Source.Node.Run(ctx, func(ctx execution.ProduceContext, record execution.Record) error {
-		getKeyHash := MakeKeyHasher(g.Source.Schema, record, g.KeyExprs)
+		getKeyHash := MakeKeyHasher(g.Source.Schema, record, g.KeyColumns)
 
 		aggColumnConsumers := make([]func(entryIndex uint, rowIndex uint), len(aggregates))
 		for i := range aggColumnConsumers {
-			aggColumnConsumers[i] = aggregates[i].MakeColumnConsumer(record.Column(g.AggregateExprs[i]))
+			aggColumnConsumers[i] = aggregates[i].MakeColumnConsumer(record.Column(g.AggregateColumns[i]))
 		}
 		newKeyAdders := make([]func(rowIndex uint), len(key))
 		for i := range newKeyAdders {
-			newKeyAdders[i] = key[i].MakeNewKeyAdder(record.Column(g.KeyExprs[i]))
+			newKeyAdders[i] = key[i].MakeNewKeyAdder(record.Column(g.KeyColumns[i]))
 		}
 		keyEqualityCheckers := make([]func(entryIndex uint, rowIndex uint) bool, len(key))
 		for i := range keyEqualityCheckers {
-			keyEqualityCheckers[i] = key[i].MakeKeyEqualityChecker(record.Column(g.KeyExprs[i]))
+			keyEqualityCheckers[i] = key[i].MakeKeyEqualityChecker(record.Column(g.KeyColumns[i]))
 		}
 
 		rows := record.NumRows()
diff --git a/arrowexec/nodes/group_by_test.go b/arrowexec/nodes/group_by_test.go
index 8a3d1cd5..1138701d 100644
--- a/arrowexec/nodes/group_by_test.go
+++ b/arrowexec/nodes/group_by_test.go
@@ -13,6 +13,7 @@ import (
 )
 
 func TestGroupBy(t *testing.T) {
+	ctx := context.Background()
 	allocator := memory.NewGoAllocator()
 
 	schema := arrow.NewSchema(
@@ -56,15 +57,15 @@ func TestGroupBy(t *testing.T) {
 			),
 			Source: node,
 
-			KeyExprs:       []int{0},
-			AggregateExprs: []int{1},
+			KeyColumns:       []int{0},
+			AggregateColumns: []int{1},
 			AggregateConstructors: []func(dt arrow.DataType) Aggregate{
 				MakeSum,
 			},
 		},
 	}
 
-	if err := node.Node.Run(context.Background(), func(ctx execution.ProduceContext, record execution.Record) error {
+	if err := node.Node.Run(execution.Context{Context: ctx}, func(ctx execution.ProduceContext, record execution.Record) error {
 		log.Println(record)
 		return nil
 	}); err != nil {
@@ -92,13 +93,13 @@ func BenchmarkGroupBy(b *testing.B) {
 		aBuilder := array.NewInt64Builder(allocator)
 		bBuilder := array.NewInt64Builder(allocator)
 
-		for i := 0; i < execution.BatchSize; i++ {
-			aBuilder.Append(int64((arrayIndex*execution.BatchSize + i) % groups))
-			bBuilder.Append(int64(arrayIndex*execution.BatchSize + i))
+		for i := 0; i < execution.IdealBatchSize; i++ {
+			aBuilder.Append(int64((arrayIndex*execution.IdealBatchSize + i) % groups))
+			bBuilder.Append(int64(arrayIndex*execution.IdealBatchSize + i))
 		}
 
 		records = append(records, execution.Record{
-			Record: array.NewRecord(schema, []arrow.Array{aBuilder.NewArray(), bBuilder.NewArray()}, execution.BatchSize),
+			Record: array.NewRecord(schema, []arrow.Array{aBuilder.NewArray(), bBuilder.NewArray()}, execution.IdealBatchSize),
 		})
 	}
 
@@ -124,8 +125,8 @@ func BenchmarkGroupBy(b *testing.B) {
 				),
 				Source: node,
 
-				KeyExprs:       []int{0},
-				AggregateExprs: []int{1},
+				KeyColumns:       []int{0},
+				AggregateColumns: []int{1},
 				AggregateConstructors: []func(dt arrow.DataType) Aggregate{
 					MakeSum,
 				},
@@ -133,7 +134,7 @@ func BenchmarkGroupBy(b *testing.B) {
 		}
 		b.StartTimer()
 
-		if err := node.Node.Run(ctx, func(ctx execution.ProduceContext, record execution.Record) error {
+		if err := node.Node.Run(execution.Context{Context: ctx}, func(ctx execution.ProduceContext, record execution.Record) error {
 			outArrays = append(outArrays, record.Record.Columns()[0])
 			return nil
 		}); err != nil {
@@ -161,7 +162,7 @@ func BenchmarkGroupByString(b *testing.B) {
 	for arrayIndex := 0; arrayIndex < rounds; arrayIndex++ {
 		aBuilder := array.NewStringBuilder(allocator)
 
-		for i := 0; i < execution.BatchSize; i++ {
+		for i := 0; i < execution.IdealBatchSize; i++ {
 			switch rand.Intn(4) {
 			case 0:
 				aBuilder.Append("aaa")
@@ -175,7 +176,7 @@ func BenchmarkGroupByString(b *testing.B) {
 		}
 
 		records = append(records, execution.Record{
-			Record: array.NewRecord(schema, []arrow.Array{aBuilder.NewArray()}, execution.BatchSize),
+			Record: array.NewRecord(schema, []arrow.Array{aBuilder.NewArray()}, execution.IdealBatchSize),
 		})
 	}
 
@@ -201,8 +202,8 @@ func BenchmarkGroupByString(b *testing.B) {
 				),
 				Source: node,
 
-				KeyExprs:       []int{0},
-				AggregateExprs: []int{0},
+				KeyColumns:       []int{0},
+				AggregateColumns: []int{0},
 				AggregateConstructors: []func(dt arrow.DataType) Aggregate{
 					MakeCount,
 				},
@@ -210,7 +211,7 @@ func BenchmarkGroupByString(b *testing.B) {
 		}
 		b.StartTimer()
 
-		if err := node.Node.Run(ctx, func(ctx execution.ProduceContext, record execution.Record) error {
+		if err := node.Node.Run(execution.Context{Context: ctx}, func(ctx execution.ProduceContext, record execution.Record) error {
 			outArrays = append(outArrays, record.Record.Columns()[0])
 			return nil
 		}); err != nil {