add method for pushing one operator under another

Signed-off-by: Andres Taylor <andres@planetscale.com>

go/vt/vtgate/planbuilder/operators/derived.go

@@ -29,7 +29,7 @@ import (
 )
 
 type Derived struct {
-	singleSource
+	*singleSource
 
 	Query         sqlparser.SelectStatement
 	Alias         string
@@ -49,7 +49,7 @@ func newDerived(
 	columnAliases sqlparser.Columns,
 ) ops.Operator {
 	return &Derived{
-		singleSource:  singleSource{Source: src},
+		singleSource:  &singleSource{Source: src},
 		Query:         stmt,
 		Alias:         alias,
 		ColumnAliases: columnAliases,
@@ -62,7 +62,7 @@ func (d *Derived) IPhysical() {}
 // Clone implements the Operator interface
 func (d *Derived) Clone(inputs []ops.Operator) ops.Operator {
 	return &Derived{
-		singleSource:  singleSource{Source: inputs[0]},
+		singleSource:  &singleSource{Source: inputs[0]},
 		Query:         d.Query,
 		Alias:         d.Alias,
 		ColumnAliases: sqlparser.CloneColumns(d.ColumnAliases),

go/vt/vtgate/planbuilder/operators/filter.go

@@ -25,7 +25,7 @@ import (
 )
 
 type Filter struct {
-	singleSource
+	*singleSource
 
 	Predicates []sqlparser.Expr
 }
@@ -34,7 +34,7 @@ var _ ops.PhysicalOperator = (*Filter)(nil)
 
 func newFilter(op ops.Operator, expr sqlparser.Expr) ops.Operator {
 	return &Filter{
-		singleSource: singleSource{Source: op}, Predicates: []sqlparser.Expr{expr},
+		singleSource: &singleSource{Source: op}, Predicates: []sqlparser.Expr{expr},
 	}
 }
 
@@ -46,7 +46,7 @@ func (f *Filter) Clone(inputs []ops.Operator) ops.Operator {
 	predicatesClone := make([]sqlparser.Expr, len(f.Predicates))
 	copy(predicatesClone, f.Predicates)
 	return &Filter{
-		singleSource: singleSource{Source: inputs[0]},
+		singleSource: &singleSource{Source: inputs[0]},
 		Predicates:   predicatesClone,
 	}
 }

go/vt/vtgate/planbuilder/operators/horizon.go

@@ -25,7 +25,7 @@ import (
 // Horizon is an operator we use until we decide how to handle the source to the horizon.
 // It contains information about the planning we have to do after deciding how we will send the query to the tablets.
 type Horizon struct {
-	singleSource
+	*singleSource
 
 	Select sqlparser.SelectStatement
 
@@ -39,7 +39,7 @@ func (h *Horizon) IPhysical() {}
 
 func newHorizon(src ops.Operator, stmt sqlparser.SelectStatement) ops.Operator {
 	return &Horizon{
-		singleSource: singleSource{src},
+		singleSource: &singleSource{src},
 		Select:       stmt,
 	}
 }
@@ -55,7 +55,7 @@ func (h *Horizon) AddPredicate(ctx *plancontext.PlanningContext, expr sqlparser.
 
 func (h *Horizon) Clone(inputs []ops.Operator) ops.Operator {
 	return &Horizon{
-		singleSource: singleSource{inputs[0]},
+		singleSource: &singleSource{inputs[0]},
 		Select:       h.Select,
 	}
 }

go/vt/vtgate/planbuilder/operators/horizon_planning.go

@@ -42,13 +42,8 @@ func planHorizons(in ops.Operator) (ops.Operator, error) {
 func planHorizon(in *Horizon) (ops.Operator, error) {
 	rb, isRoute := in.Source.(*Route)
 	if isRoute && rb.IsSingleShard() && in.Select.GetLimit() == nil {
-		return planSingleShardRoute(rb, in)
+		return swap(in, rb), nil
 	}
 
 	return in, nil
 }
-
-func planSingleShardRoute(rb *Route, horizon *Horizon) (ops.Operator, error) {
-	rb.Source, horizon.Source = horizon, rb.Source
-	return rb, nil
-}

go/vt/vtgate/planbuilder/operators/operator.go

@@ -54,6 +54,12 @@ type (
 	singleSource struct {
 		Source ops.Operator
 	}
+
+	singleSourceI interface {
+		ops.Operator
+		getSource() ops.Operator
+		setSource(ops.Operator)
+	}
 )
 
 func PlanQuery(ctx *plancontext.PlanningContext, selStmt sqlparser.Statement) (ops.Operator, error) {
@@ -89,10 +95,18 @@ func (noInputs) Inputs() []ops.Operator {
 }
 
 // Inputs implements the Operator interface
-func (s singleSource) Inputs() []ops.Operator {
+func (s *singleSource) Inputs() []ops.Operator {
 	return []ops.Operator{s.Source}
 }
 
+func (s *singleSource) getSource() ops.Operator {
+	return s.Source
+}
+
+func (s *singleSource) setSource(src ops.Operator) {
+	s.Source = src
+}
+
 // AddColumn implements the Operator interface
 func (noColumns) AddColumn(*plancontext.PlanningContext, sqlparser.Expr) (int, error) {
 	return 0, vterrors.Errorf(vtrpcpb.Code_INTERNAL, "this operator cannot accept columns")
@@ -102,3 +116,13 @@ func (noColumns) AddColumn(*plancontext.PlanningContext, sqlparser.Expr) (int, e
 func (noPredicates) AddPredicate(*plancontext.PlanningContext, sqlparser.Expr) (ops.Operator, error) {
 	return nil, vterrors.Errorf(vtrpcpb.Code_INTERNAL, "this operator cannot accept predicates")
 }
+
+// swap takes two operators with a single source and swaps them. the assumption is that the first op provided has the
+// second operator as source. this method will the two operators with each other and return the new root.
+// this method is used to push one operator underneath the other
+func swap(op, src singleSourceI) ops.Operator {
+	tmp := src.getSource()
+	src.setSource(op)
+	op.setSource(tmp)
+	return src
+}

go/vt/vtgate/planbuilder/operators/route.go

@@ -33,7 +33,7 @@ import (
 
 type (
 	Route struct {
-		singleSource
+		*singleSource
 
 		RouteOpCode engine.Opcode
 		Keyspace    *vindexes.Keyspace
@@ -99,7 +99,7 @@ func newRoute(
 	sysTableTableName map[string]evalengine.Expr,
 ) *Route {
 	return &Route{
-		singleSource:        singleSource{src},
+		singleSource:        &singleSource{src},
 		RouteOpCode:         opCode,
 		Keyspace:            keyspace,
 		VindexPreds:         vindexPreds,
@@ -140,7 +140,7 @@ func (r *Route) Cost() int {
 // Clone implements the Operator interface
 func (r *Route) Clone(inputs []ops.Operator) ops.Operator {
 	cloneRoute := *r
-	cloneRoute.singleSource = singleSource{inputs[0]}
+	cloneRoute.singleSource = &singleSource{inputs[0]}
 	cloneRoute.VindexPreds = make([]*VindexPlusPredicates, len(r.VindexPreds))
 	for i, pred := range r.VindexPreds {
 		// we do this to create a copy of the struct

go/vt/vtgate/planbuilder/operators/route_planning.go

@@ -89,9 +89,7 @@ func transformToPhysical(ctx *plancontext.PlanningContext, in ops.Operator) (ops
 func optimizeFilter(op *Filter) (ops.Operator, rewrite.TreeIdentity, error) {
 	if route, ok := op.Source.(*Route); ok {
 		// let's push the filter into the route
-		op.Source = route.Source
-		route.Source = op
-		return route, rewrite.NewTree, nil
+		return swap(op, route), rewrite.NewTree, nil
 	}
 
 	return op, rewrite.SameTree, nil