dolthub/dolt#9807 - FULL OUTER JOIN with empty left subquery rets unmatched right rows

enginetest/join_op_tests.go

@@ -105,6 +105,61 @@ var EngineOnlyJoinOpTests = []joinOpTest{
 }
 
 var DefaultJoinOpTests = []joinOpTest{
+	{
+		// https://github.com/dolthub/dolt/issues/9807
+		name: "FULL OUTER JOIN fails with empty subquery",
+		setup: [][]string{
+			{
+				"CREATE TABLE t(c BOOLEAN);",
+				"INSERT INTO t VALUES (FALSE);",
+			},
+		},
+		tests: []JoinOpTests{
+			{
+				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},
+				},
+			},
+		},
+	},
+	{
+		// https://github.com/dolthub/dolt/issues/9807
+		name: "FULL OUTER JOIN with empty tables",
+		setup: [][]string{
+			{
+				"CREATE TABLE t1 (i INT);",
+				"CREATE TABLE t2 (j INT);",
+				"INSERT INTO t2 VALUES (1);",
+			},
+		},
+		tests: []JoinOpTests{
+			{
+				Query: "SELECT i, j FROM t1 FULL OUTER JOIN t2 ON 1=1;",
+				Expected: []sql.Row{
+					{nil, 1},
+				},
+			},
+			{
+				Query: "SELECT j, i FROM t2 FULL OUTER JOIN t1 ON 1=1;",
+				Expected: []sql.Row{
+					{1, nil},
+				},
+			},
+			{
+				Query: "SELECT i, j FROM t1 FULL OUTER JOIN t2 ON 1=0;",
+				Expected: []sql.Row{
+					{nil, 1},
+				},
+			},
+			{
+				Query: "SELECT j, i FROM t2 FULL OUTER JOIN t1 ON 1=0;",
+				Expected: []sql.Row{
+					{1, nil},
+				},
+			},
+		},
+	},
 	{
 		name: "bug where transitive join edge drops filters",
 		setup: [][]string{

sql/rowexec/join_iters.go

@@ -487,6 +487,10 @@ 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 +516,9 @@ 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)
+			// Phase 2 of FULL OUTER JOIN: return unmatched right rows as (null, rightRow).
+			// Use parentRow instead of leftRow since leftRow is nil when left side is empty.
+			iter, err := i.b.Build(ctx, i.rp, i.parentRow)
 			if err != nil {
 				return nil, err
 			}