add fix for 9807

Author: elianddb

enginetest/queries/join_queries.go

@@ -792,6 +792,22 @@ on w = 0;`,
 }
 
 var JoinScriptTests = []ScriptTest{
+	{
+		// https://github.com/dolthub/dolt/issues/9807
+		Name: "FULL OUTER JOIN fails with empty subquery",
+		SetUpScript: []string{
+			"CREATE TABLE t(c BOOLEAN);",
+			"INSERT INTO t VALUES (FALSE);",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: "SELECT * FROM ( SELECT c FROM t WHERE c ) sub1 FULL OUTER JOIN ( SELECT 1 AS c ) sub2 ON 1=1;",
+				Expected: []sql.Row{
+					{nil, 1},
+				},
+			},
+		},
+	},
 	{
 		Name:        "Simple join query",
 		SetUpScript: []string{},

sql/rowexec/join_iters.go

@@ -20,6 +20,7 @@ import (
 	"io"
 	"reflect"
 
+	"github.com/sirupsen/logrus"
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/trace"
 
@@ -405,6 +406,7 @@ func (i *existsIter) Close(ctx *sql.Context) (err error) {
 }
 
 func newFullJoinIter(ctx *sql.Context, b sql.NodeExecBuilder, j *plan.JoinNode, row sql.Row) (sql.RowIter, error) {
+	logrus.Info("full_join_prefix: Creating new full join iterator")
 	leftIter, err := b.Build(ctx, j.Left(), row)
 	if err != nil {
 		return nil, err
@@ -447,11 +449,13 @@ type fullJoinIter struct {
 func (i *fullJoinIter) Next(ctx *sql.Context) (sql.Row, error) {
 	for {
 		if i.leftDone {
+			logrus.Info("full_join_prefix: Left side finished, entering right-only phase")
 			break
 		}
 		if i.leftRow == nil {
 			r, err := i.l.Next(ctx)
 			if errors.Is(err, io.EOF) {
+				logrus.Info("full_join_prefix: Left side exhausted, no more left rows")
 				i.leftDone = true
 				i.l = nil
 				i.r = nil
@@ -487,6 +491,10 @@ func (i *fullJoinIter) Next(ctx *sql.Context) (sql.Row, error) {
 			}
 			i.r = nil
 			i.leftRow = nil
+			continue
+		}
+		if err != nil {
+			return nil, err
 		}
 
 		row := i.buildRow(i.leftRow, rightRow)
@@ -512,7 +520,8 @@ func (i *fullJoinIter) Next(ctx *sql.Context) (sql.Row, error) {
 
 	for {
 		if i.r == nil {
-			iter, err := i.b.Build(ctx, i.rp, i.leftRow)
+			logrus.Info("full_join_prefix: Building right iterator for unmatched right rows")
+			iter, err := i.b.Build(ctx, i.rp, i.parentRow)
 			if err != nil {
 				return nil, err
 			}