INSERT ... VALUES... with ORDER BY / LIMIT

While tackling the top-level CTEs patch, I found that INSERT ...
VALUES isn't aware of ORDER BY / LIMIT.

regression=# CREATE TABLE t1(x int);
CREATE TABLE
regression=# INSERT INTO t1 VALUES (1),(2),(3) LIMIT 1;
INSERT 0 3
regression=# TABLE t1;
x
---
1
2
3
(3 rows)

regression=# TRUNCATE t1;
TRUNCATE TABLE
regression=# INSERT INTO t1 VALUES (1),(2),(3) ORDER BY 2;
INSERT 0 3
regression=# TABLE t1;
x
---
1
2
3
(3 rows)

Is it intentional, or a bug?

This behavior gives me a thought that current INSERT ... VALUES should
be refactored. Query(INSERT)->rtable has a RangeTblEntry(RTE_VALUES)
as the representation of VALUES, but to be consistent with the syntax,
rtable should have RangeTblEntry(RTE_SUBSELECT), whose rtable member
holds RangeTblEntry(RTE_VALUES) as the normal SELECT does. I see
there's some reason here why INSERT case doesn't handle it as the
normal SELECT does, and I don't see how big this change affect to
around rewriter and planner. On the other hand, if INSERT ... VALUES
doesn't care about ORDER BY / LIMIT, it should ignore WITH clause as
well. Ignoring WITH breaks the current behavior, but the use case is
quite narrow and I bet no one relies on it, so we might be able to
break it.

Regards,

--
Hitoshi Harada