vam: Support NullScan (#5613)

Author: mattnibs

compiler/kernel/vop.go

@@ -202,6 +202,9 @@ func (b *Builder) compileVamLeaf(o dag.Op, parent vector.Puller) (vector.Puller,
 		return vamop.NewFilter(b.zctx(), parent, e), nil
 	case *dag.Head:
 		return vamop.NewHead(parent, o.Count), nil
+	case *dag.NullScan:
+		return vam.NewDematerializer(zbuf.NewPuller(zbuf.NewArray([]super.Value{super.Null}))), nil
+
 	case *dag.Output:
 		b.channels[o.Name] = append(b.channels[o.Name], vam.NewMaterializer(parent))
 		return parent, nil

vector/any.go

@@ -20,3 +20,20 @@ type Promotable interface {
 type Puller interface {
 	Pull(done bool) (Any, error)
 }
+
+type puller struct {
+	vecs []Any
+}
+
+func NewPuller(vecs ...Any) Puller {
+	return &puller{vecs}
+}
+
+func (p *puller) Pull(_ bool) (Any, error) {
+	if len(p.vecs) == 0 {
+		return nil, nil
+	}
+	vec := p.vecs[0]
+	p.vecs = p.vecs[1:]
+	return vec, nil
+}