Small fix to decrease memory allocation (#16791)

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

go/vt/vtgate/planbuilder/operators/delete.go

@@ -52,8 +52,8 @@ func (d *Delete) GetOrdering(*plancontext.PlanningContext) []OrderBy {
 	return nil
 }
 
-func (d *Delete) TablesUsed() []string {
-	return SingleQualifiedIdentifier(d.Target.VTable.Keyspace, d.Target.VTable.Name)
+func (d *Delete) TablesUsed(in []string) []string {
+	return append(in, QualifiedString(d.Target.VTable.Keyspace, d.Target.VTable.Name.String()))
 }
 
 func (d *Delete) ShortDescription() string {

go/vt/vtgate/planbuilder/operators/helpers.go

@@ -18,6 +18,7 @@ package operators
 
 import (
 	"fmt"
+	"slices"
 	"sort"
 
 	"vitess.io/vitess/go/vt/sqlparser"
@@ -82,20 +83,21 @@ func TableID(op Operator) (result semantics.TableSet) {
 
 // TableUser is used to signal that this operator directly interacts with one or more tables
 type TableUser interface {
-	TablesUsed() []string
+	TablesUsed([]string) []string
 }
 
 func TablesUsed(op Operator) []string {
-	addString, collect := collectSortedUniqueStrings()
+	var in []string
 	_ = Visit(op, func(this Operator) error {
 		if tbl, ok := this.(TableUser); ok {
-			for _, u := range tbl.TablesUsed() {
-				addString(u)
-			}
+			in = tbl.TablesUsed(in)
 		}
 		return nil
 	})
-	return collect()
+
+	slices.Sort(in)
+	compacted := slices.Compact(in)
+	return compacted
 }
 
 func CostOf(op Operator) (cost int) {

go/vt/vtgate/planbuilder/operators/insert.go

@@ -96,8 +96,12 @@ func (i *Insert) Clone([]Operator) Operator {
 	}
 }
 
-func (i *Insert) TablesUsed() []string {
-	return SingleQualifiedIdentifier(i.VTable.Keyspace, i.VTable.Name)
+func (i *Insert) TablesUsed(in []string) []string {
+	return append(in, i.tableTarget())
+}
+
+func (i *Insert) tableTarget() string {
+	return QualifiedString(i.VTable.Keyspace, i.VTable.Name.String())
 }
 
 func (i *Insert) Statement() sqlparser.Statement {
@@ -423,7 +427,7 @@ func insertSelectPlan(
 	// When the table you are streaming data from and table you are inserting from are same.
 	// Then due to locking of the index range on the table we might not be able to insert into the table.
 	// Therefore, instead of streaming, this flag will ensure the records are first read and then inserted.
-	insertTbl := insOp.TablesUsed()[0]
+	insertTbl := insOp.tableTarget()
 	selTables := TablesUsed(selOp)
 	for _, tbl := range selTables {
 		if insertTbl == tbl {

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

@@ -717,14 +717,11 @@ func (r *Route) GetOrdering(ctx *plancontext.PlanningContext) []OrderBy {
 
 // TablesUsed returns tables used by MergedWith routes, which are not included
 // in Inputs() and thus not a part of the operator tree
-func (r *Route) TablesUsed() []string {
-	addString, collect := collectSortedUniqueStrings()
+func (r *Route) TablesUsed(in []string) []string {
 	for _, mw := range r.MergedWith {
-		for _, u := range TablesUsed(mw) {
-			addString(u)
-		}
+		in = append(in, TablesUsed(mw)...)
 	}
-	return collect()
+	return in
 }
 
 func isSpecialOrderBy(o OrderBy) bool {

go/vt/vtgate/planbuilder/operators/table.go

@@ -107,11 +107,11 @@ func (to *Table) AddCol(col *sqlparser.ColName) {
 	to.Columns = append(to.Columns, col)
 }
 
-func (to *Table) TablesUsed() []string {
+func (to *Table) TablesUsed(in []string) []string {
 	if sqlparser.SystemSchema(to.QTable.Table.Qualifier.String()) {
-		return nil
+		return in
 	}
-	return SingleQualifiedIdentifier(to.VTable.Keyspace, to.VTable.Name)
+	return append(in, QualifiedString(to.VTable.Keyspace, to.VTable.Name.String()))
 }
 
 func addColumn(ctx *plancontext.PlanningContext, op ColNameColumns, e sqlparser.Expr) int {

go/vt/vtgate/planbuilder/operators/update.go

@@ -86,8 +86,8 @@ func (u *Update) GetOrdering(*plancontext.PlanningContext) []OrderBy {
 	return nil
 }
 
-func (u *Update) TablesUsed() []string {
-	return SingleQualifiedIdentifier(u.Target.VTable.Keyspace, u.Target.VTable.Name)
+func (u *Update) TablesUsed(in []string) []string {
+	return append(in, QualifiedString(u.Target.VTable.Keyspace, u.Target.VTable.Name.String()))
 }
 
 func (u *Update) ShortDescription() string {

go/vt/vtgate/planbuilder/operators/vindex.go

@@ -164,8 +164,8 @@ func (v *Vindex) AddPredicate(ctx *plancontext.PlanningContext, expr sqlparser.E
 
 // TablesUsed implements the Operator interface.
 // It is not keyspace-qualified.
-func (v *Vindex) TablesUsed() []string {
-	return []string{v.Table.Table.Name.String()}
+func (v *Vindex) TablesUsed(in []string) []string {
+	return append(in, v.Table.Table.Name.String())
 }
 
 func (v *Vindex) ShortDescription() string {