Re: Changed SRF in targetlist handling

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, May 23, 2016 at 4:15 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> 2. Rewrite into LATERAL ROWS FROM (srf1(), srf2(), ...). This would
>> have the same behavior as before if the SRFs all return the same number
>> of rows, and otherwise would behave differently.

> I thought the idea was to rewrite it as LATERAL ROWS FROM (srf1()),
> LATERAL ROWS FROM (srf2()), ...

No, because then you get the cross-product of multiple SRFs, not the
run-in-lockstep behavior.

> The rewrite you propose here seems to NULL-pad rows after the first
> SRF is exhausted:

Yes. That's why I said it's not compatible if the SRFs don't all return
the same number of rows. It seems like a reasonable definition to me
though, certainly much more reasonable than the current run-until-LCM
behavior.

> The latter is how I'd expect SRF-in-targetlist to work.

That's not even close to how it works now. It would break *every*
existing application that has multiple SRFs in the tlist, not just
the ones whose SRFs return different numbers of rows. And I'm not
convinced that it's a more useful behavior.

regards, tom lane