Re: WIP: Covering + unique indexes.

On Fri, Mar 30, 2018 at 6:24 AM, Alexander Korotkov
<a(dot)korotkov(at)postgrespro(dot)ru> wrote:
>> With an extreme enough case, this could result in a failure to find a
>> split point. Or at least, if that isn't true then it's not clear why,
>> and I think it needs to be explained.
>
>
> I don't think this could result in a failure to find a split point.
> So, it finds a split point without taking into account that hikey
> will be shorter. If such split point exists then split point with
> truncated hikey should also exists. If not, then it would be
> failure even without covering indexes. I've updated comment
> accordingly.

You're right. We're going to truncate the unneeded trailing attributes
from whatever tuple is to the immediate right of the final split point
that we choose (that's the tuple that we'll copy to make a new high
key for the left page). Truncation already has to result in a tuple
that is less than or equal to the original tuple.

I also agree that it isn't worth trying harder to make sure that space
is distributed evenly when truncation will go ahead. It will only
matter in very rare cases, but the computational overhead of having an
accurate high key size for every candidate split point would be
significant.

--
Peter Geoghegan