Re: Named column in a Function fails at ORDER BY (PgSQL 7.1)

Bernd von den Brincken <bvdb@asa.de> writes:
> on the occasion of a subselect I found a behavior in PostgreSQL 7.1 that I 
> consider faulty, but would like to hear comments before complaining to the 
> hackers:

Too late ;-)

> Here is the example - this fails with "ERROR: Attribute 'max_date' not
> found": 

>     SELECT content, datetime AS max_date
>               FROM cftext
>             ORDER by date_part('hour', max_date)

> Whereas this works both:

> a)     SELECT content, datetime AS max_date
>               FROM cftext
>             ORDER by  max_date

> b)    SELECT content, datetime AS max_date
>               FROM cftext
>             ORDER by date_part('hour', datetime)

Right.  The SQL92 spec disallows expressions in ORDER BY, and states that
the ORDER BY items are either names or numbers of *output columns* of
the SELECT.  Thus, only your alternative (a) is legal per spec.
Postgres chooses to also allow sorting on expressions that are not in
the output list --- but such expressions are considered to be additional
expressions over the input columns.  Essentially it's a way to write
SELECT content,              datetime AS max_date,              date_part('hour', datetime) AS order_valFROM
cftextORDERBY order_val
 

(which would be a spec-legal construct) and then tell the system you
didn't really want to see the order_val column in your output.

SQL99 seems to have (incompatibly) redefined ORDER BY to allow
expressions over the output column names, but I don't have a lot of
appetite for breaking existing applications in order to conform to the
SQL99 definition.

I do not have any sympathy for the notion that we should accept either
input or output column names in such expressions, if that's what you
were hoping to suggest.  It's ambiguous.

> Now in this example the alternative works fine, but if you use a Sub-SELECT 
> there is no alternative to a named result column.

Why not?  You can always do something like
SELECT * FROM (SELECT ... ) AS TORDER BY x;

which is legal under both SQL specs.
        regards, tom lane