Re: ALTER TABLE ADD COLUMN fast default

On 2018-03-29 16:40:29 -0700, Andres Freund wrote:
> Hi,
>
> On 2018-03-30 10:08:54 +1030, Andrew Dunstan wrote:
> > On Fri, Mar 30, 2018 at 5:00 AM, Andres Freund <andres@anarazel.de> wrote:
> > > this'll trigger one slot_getsomeattrs(slot, 2) call from within qual
> > > evaluation, and then a slot_getsomeattrs(slot, 4) from within the
> > > projection code.  If you then imagine a tuple where only the first
> > > column exists physically, we'd copy b twice, because attno is only going
> > > to be one 1.  I think we might just want to check tts nvalid in
> > > getmissingattrs?
>
> > OK. so add something like this to the top of slot_getmissingattrs()?
> >
> >     startAttNum = Max(startAttNum, slot->tts_nvalid);
>
> Yea, I think that should do the trick.

Hm, or if you want to microoptimize ;):

if (startAttNum < slot->tts_nvalid)
   startAttNum = slot->tts_nvalid

I think that can use cmov (i.e. no visible branch), which Max() probably
can't usefully. Don't think the compiler can figure out that
slot->tts_nvalid cannot be smaller than startAttNum.

Greetings,

Andres Freund