Re: WIP: Covering + unique indexes.

On Wed, Apr 6, 2016 at 6:15 AM, Anastasia Lubennikova
<a.lubennikova@postgrespro.ru> wrote:
>> * I would like to see index_reform_tuple() assert that the new,
>> truncated index tuple is definitely <= the original (I worry about the
>> 1/3 page restriction issue). Maybe you should also change the name of
>> index_reform_tuple(), per David.
>
> Is it possible that the new tuple, containing less attributes than the old
> one, will have a greater size?
> Maybe you can give an example?
> I think that  Assert(indnkeyatts <= indnatts); covers this kind of errors.

I don't think it is possible, because you aren't e.g. making an
attribute's value NULL where it wasn't NULL before (making the
IndexTuple contain a NULL bitmap where it didn't before). But that's
kind of subtle, and it certainly seems worth an assertion. It could
change tomorrow, when someone optimizes heap_deform_tuple(), which has
been proposed more than once.

Personally, I like documenting assertions, and will sometimes write
assertions that the compiler could easily optimize away. Maybe going
*that* far is more a matter of personal style, but I think an
assertion about the new index tuple size being <= the old one is just
a good idea. It's not about a problem in your code at all.

> I do not mind to rename this function, but what name would be better?
> index_truncate_tuple()?

That seems better, yes.

-- 
Peter Geoghegan