XMAX weirdness (was: Plans for solving the VACUUM problem)

Bruce Momjian wrote:
> 
> You can read my Internals paper at the bottom of developers corner.
> 

in answer to my question:

> > BTW, is there some place where I could read about exact semantics of
> > sytem fields ?

the only thing I found was on page 50 which claimed that

xmax - destruction transaction id

which is what I remembered, 

but then no xmax should ever be visible in a regular query :


hannu=# create table parent(pid int primary key);
NOTICE:  CREATE TABLE/PRIMARY KEY will create implicit index
'parent_pkey' for table 'parent'
CREATE
hannu=# create table child(cid int references parent(pid) on update
cascade);
NOTICE:  CREATE TABLE will create implicit trigger(s) for FOREIGN KEY
check(s)
CREATE
hannu=# insert into parent values(1);
INSERT 27525 1
hannu=# insert into child values(1);
INSERT 27526 1
hannu=# update parent set pid=2;
UPDATE 1
hannu=# select xmin,xmax,* from parent;xmin | xmax | pid 
------+------+----- 688 |  688 |   2
(1 row)

------------------------
Hannu