Re: Implement targetlist SRFs using ROWS FROM() (was Changed SRF in targetlist handling)

On 2016-09-02 09:05:35 -0500, Kevin Grittner wrote:
> On Fri, Sep 2, 2016 at 3:31 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > On Tue, Aug 23, 2016 at 3:10 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> >> =# SELECT * FROM few, ROWS FROM(generate_series(1,3));
> >> ┌────┬─────────────────┐
> >> │ id │ generate_series │
> >> ├────┼─────────────────┤
> >> │ 1 │ 1 │
> >> │ 2 │ 1 │
> >> │ 1 │ 2 │
> >> │ 2 │ 2 │
> >> │ 1 │ 3 │
> >> │ 2 │ 3 │
> >> └────┴─────────────────┘
> >> (6 rows)
> >> surely isn't what was intended. So the join order needs to be enforced.
> >
> > In general, we've been skeptical about giving any guarantees about
> > result ordering.

Well, it's historically how we behaved for SRFs. I'm pretty sure that
people will be confused if
SELECT generate_series(1, 10) FROM sometbl;
suddenly returns rows in an order that reverse from what
generate_series() returns.

> +
>
> I think it is a very bad idea to move away from the statement that
> a query generates a set of rows, and that no order is guaranteed
> unless the top level has an ORDER BY clause. How hard is it to add
> ORDER BY 1, 2 to the above query? Let the optimizer notice when a
> node returns data in the needed order and skip the sort if possible.

There's no such infrastructure for SRFS/ROWS FROM.

Andres