Re: SP-GiST bug.

> I don't think that checkAllTheSame is broken, but it probably would be
> after this patch --- ISTM you're breaking the corner case described in the
> header comment for checkAllTheSame, where we need to force splitting of a
> set of existing tuples that the opclass can't distinguish.
INHO, this patch will not break - because it forces (in corner case) to create a
node for new tuple but exclude it from list to insert. On next iteration we will
find a needed node with empty list.

Comment:
* If we know that the leaf tuples wouldn't all fit on one page, then we
* exclude the last tuple (which is the incoming new tuple that forced a split)
* from the check to see if more than one node is used. The reason for this
* is that if the existing tuples are put into only one chain, then even if
* we move them all to an empty page, there would still not be room for the
* new tuple, so we'd get into an infinite loop of picksplit attempts.

If we reserve a node for new tuple then on next iteration we will not fall into
picksplit again - we will have an empty list. So new tuple will fall into
another page.

BTW, look for following example:
create table xxx (p point);

insert into xxx select '(1,1)'::point from generate_series(1, 226) as v;
insert into xxx values ('(3,3)'::point);

create index xxxidx on xxx using spgist (p quad_point_ops);

easy to see that the single node is allTheSame although underlying leafs are
different. Due to allTheSame search logic scans are correct but inefficient. And
patch fixes it too.

> What actually is broken, I think, is the spgist text opclass, because it's
> using a zero node label for two different situations. The scenario we're
> looking at here is:

Agree for different situations, but disagree that's broken :)

> It seems like we have to
> distinguish the case of zero-length string from that of a dummy label
> for a pushed-down allTheSame tuple.

Actually, we do this: if allTheSame is true then nodes have a dummy labels.

--
Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru
WWW: http://www.sigaev.ru/