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