Re: WIP: Covering + unique indexes.

On Tue, Mar 27, 2018 at 10:14 AM, Teodor Sigaev <teodor(at)sigaev(dot)ru> wrote:
>> b) I don't like an idea to limiting usage of that field if we can do not
>> that. Future usage could use it, for example, for different compression
>> technics or something else.Or even removing t_tid from inner tuples to save
>> 2 bytes in IndexTupleData. Of
>
> course, I remember about aligment, but it could be subject to change too in
> future.

This is contradictory. You seem to be arguing that we need to preserve
on-disk compatibility for an optimization that throws out on-disk
compatibility.

Saving a single byte per internal IndexTuple is not worth it. We could
actually save 2 bytes in *all* nbtree pages, by halving the size of
ItemId for nbtree -- we don't need lp_len, which is redundant, and we
could reclaim one of the status bits too, to get back a full 16 bits.
Also, we could use suffix truncation to save at least one byte in
almost all cases, even with the thinnest possible
single-integer-attribute IndexTuples. What you describe just isn't
going to happen.

--
Peter Geoghegan