Questions of 'for update'

Hi,

I am reading the code that generating plan for `rowmarks` of Postgres
9.4 (
https://github.com/postgres/postgres/blob/REL9_4_STABLE/src/backend/optimizer/plan/planner.c#L2070
)

After emitting the `LockRows` plannode, the results cannot be considered
in order, and there are comments there:

/*
* The result can no longer be assumed sorted, since locking might
* cause the sort key columns to be replaced with new values.
*/

I do not understand the reason and after some guess, I come up with a case:

```
create table t(c int);
insert into t values (1), (2), (3), (4);

-- Transaction 1
begin;
update t set c = 999 where c = 1; -- change the smallest value to a very
big one
-- transaction 1 not commit yet

-- Transaction 2, another session
begin;
select * from t order by c limit 1 for update; -- Want to find the smallest
value, and then update it
-- this transaction will be blocked by transaction 1

-- then, transaction 1 commit and transaction 2 will return the tuple with
value 999
```

I think the reason is that EvalPlanQual does not check the order.

I try this case under mysql, it will output 2 (which is the correct value
for the meaning of smallest).

So, in summary, my questions are:

1. why after emitting `lockrows` plannode, the result can no longer be
assumed sorted?
2. Is the case above a bug or a feature?

Thanks!

Best Regards,
Zhenghua Lyu