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
incomingnew tuple that forced a split) * from the check to see if more than one node is used.  The reason for this * is
thatif the existing tuples are put into only one chain, then even if * we move them all to an empty page, there would
stillnot 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
allTheSametuple.
 

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


-- 
Teodor Sigaev                                   E-mail: teodor@sigaev.ru
  WWW: http://www.sigaev.ru/