do not convert full outer joins to cross joins

Author: angelamayxie

enginetest/join_op_tests.go

@@ -2181,6 +2181,7 @@ WHERE
 	},
 	{
 		// https://github.com/dolthub/dolt/issues/9782
+		// https://github.com/dolthub/dolt/issues/9973
 		name: "joining with subquery on empty table",
 		setup: [][]string{
 			{
@@ -2205,6 +2206,14 @@ WHERE
 				Query:    "SELECT t.c FROM (SELECT t.c FROM t WHERE FALSE) AS subq NATURAL RIGHT JOIN t;",
 				Expected: []sql.Row{{1}},
 			},
+			{
+				Query:    "SELECT t.c FROM t FULL OUTER JOIN (SELECT t.c FROM t WHERE FALSE) AS subq ON TRUE;",
+				Expected: []sql.Row{{1}},
+			},
+			{
+				Query:    "SELECT t.c FROM (SELECT t.c FROM t WHERE FALSE) AS subq FULL OUTER JOIN t ON TRUE;",
+				Expected: []sql.Row{{1}},
+			},
 		},
 	},
 	{

sql/planbuilder/from.go

@@ -79,10 +79,13 @@ func (b *Builder) isUsingJoin(te *ast.JoinTableExpr) bool {
 }
 
 func (b *Builder) canConvertToCrossJoin(te *ast.JoinTableExpr) bool {
-	return !strings.EqualFold(te.Join, ast.LeftJoinStr) &&
-		!strings.EqualFold(te.Join, ast.RightJoinStr) &&
-		(te.Condition.On == nil || te.Condition.On == ast.BoolVal(true)) &&
-		te.Condition.Using == nil
+	switch te.Join {
+	case ast.LeftJoinStr, ast.RightJoinStr, ast.FullOuterJoinStr:
+		return false
+	default:
+		return (te.Condition.On == nil || te.Condition.On == ast.BoolVal(true)) &&
+			te.Condition.Using == nil
+	}
 }
 
 func (b *Builder) buildJoin(inScope *scope, te *ast.JoinTableExpr) (outScope *scope) {