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

On Wed, Jan 18, 2017 at 4:14 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> I wrote:
> > I'll try to write something about the SRF-in-CASE issue too. Seeing
> > whether we can document that adequately seems like an important part
> > of making the decision about whether we need to block it.
>
> Here's what I came up with:
>
> This behavior also means that set-returning functions will be evaluated
> even when it might appear that they should be skipped because of a
> conditional-evaluation construct, such as CASE or COALESCE. For example,
> consider
>
> SELECT x, CASE WHEN x > 0 THEN generate_series(1, 5) ELSE 0 END FROM tab;
>
> It might seem that this should produce five repetitions of input rows
> that have x > 0, and a single repetition of those that do not; but
> actually it will produce five repetitions of every input row.
>
> So is this too ugly to live, or shall we put up with it?
>
>
​Disallowing such an unlikely, and un-intuitive, corner-case strikes my
sensibilities.

​I'd rather fail now and allow for the possibility of future implementation
of the "it might seem that..." behavior.​

David J.