build update using interfaces

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

go/vt/sqlparser/ast.go

@@ -716,6 +716,10 @@ type (
 	IndexType int8
 )
 
+var _ OrderAndLimit = (*Select)(nil)
+var _ OrderAndLimit = (*Update)(nil)
+var _ OrderAndLimit = (*Delete)(nil)
+
 func (*Union) iStatement()               {}
 func (*Select) iStatement()              {}
 func (*Stream) iStatement()              {}

go/vt/sqlparser/ast_funcs.go

@@ -2622,3 +2622,43 @@ func (node *Update) AddFrom(tbl TableExpr) {
 func (node *Delete) AddFrom(tbl TableExpr) {
 	node.TableExprs = append(node.TableExprs, tbl)
 }
+
+func (node *Update) AddOrder(order *Order) {
+	node.OrderBy = append(node.OrderBy, order)
+}
+
+func (node *Update) SetOrderBy(by OrderBy) {
+	node.OrderBy = by
+}
+
+func (node *Update) GetOrderBy() OrderBy {
+	return node.OrderBy
+}
+
+func (node *Update) GetLimit() *Limit {
+	return node.Limit
+}
+
+func (node *Update) SetLimit(limit *Limit) {
+	node.Limit = limit
+}
+
+func (node *Delete) AddOrder(order *Order) {
+	node.OrderBy = append(node.OrderBy, order)
+}
+
+func (node *Delete) SetOrderBy(by OrderBy) {
+	node.OrderBy = by
+}
+
+func (node *Delete) GetOrderBy() OrderBy {
+	return node.OrderBy
+}
+
+func (node *Delete) GetLimit() *Limit {
+	return node.Limit
+}
+
+func (node *Delete) SetLimit(limit *Limit) {
+	node.Limit = limit
+}

go/vt/vtgate/planbuilder/operators/SQL_builder.go

@@ -411,26 +411,13 @@ func buildDelete(op *Delete, qb *queryBuilder) {
 
 func buildUpdate(op *Update, qb *queryBuilder) {
 	updExprs := getUpdateExprs(op)
-
-	buildQuery(op.Source, qb)
-	// currently the qb builds a select query underneath.
-	// Will take the `From` and `Where` from this select
-	// and create a update statement.
-	// TODO: change it to directly produce `update` statement.
-	sel, ok := qb.stmt.(*sqlparser.Select)
-	if !ok {
-		panic(vterrors.VT13001("expected a select here"))
+	upd := &sqlparser.Update{
+		Ignore: op.Ignore,
+		Exprs:  updExprs,
 	}
-
+	qb.stmt = upd
 	qb.dmlOperator = op
-	qb.stmt = &sqlparser.Update{
-		Ignore:     op.Ignore,
-		TableExprs: sel.From,
-		Exprs:      updExprs,
-		Where:      sel.Where,
-		Limit:      sel.Limit,
-		OrderBy:    sel.OrderBy,
-	}
+	buildQuery(op.Source, qb)
 }
 
 func getUpdateExprs(op *Update) sqlparser.UpdateExprs {