select a.name ... union select a.name ... order by a.name fails in 7.1

Palle Girgensohn (girgen@partitur.se) reports a bug with a severity of 2
The lower the number the more severe it is.

Short Description
select a.name ... union select a.name ... order by a.name fails in 7.1

Long Description
Hi!

I'm not certain this is correct SQL, and I know the rewrite is
easy, but here is a difference I found between 7.0.3 and 7.1:

select a.name from users a
union
select a.name from oldusers a
order by a.userid;

ERROR:  Relation 'a' does not exist

This works fine in postgres 7.0.x

the simple rewrite is of course 'order by userid', but it is
tedious to find all places in our system where this happens. It
seems some programmers have used the above syntax a lot :(

Is it a bug, or was it a feature in 7.0?

/Palle

Sample Code
fails on 7.1, work on 7.0.3:

select a.name from users a
union
select a.name from oldusers a
order by a.userid;


modifying last line ot look like 'order by userid' fixes the problem.

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