thanks for your clarification.
> You need to look at it (XMIN) in conjunction with XMAX. A newly insert row has
> XMIN set and XMAX null. When a row is updated the XMAX of the old row
> is set and a new row is created with an XMIN. When you delete a row it
> just sets the XMAX.
But, as you say below, it also "disappears"... :-)
> > IOW, can I also detect my row being *deleted* from under me by
> > another transaction by way of checking XMIN ? Else I would
> > likely need to check XMAX, too.
> Easy, look for it. If you can't find it, it got deleted...
Doh, of course you are right. I was thinking of doing this:
(assume a row with pk set to 1)
select xmin, ... from ... where pk=1;
... remember xmin as <old_xmin> ...
... do some time-intensive application work ...
select 1 from ... where pk=1 and xmin=<old_xmin> for update;
- if one row (eg. the "1") is returned then I locked my row
and can happily update it and commit
- if more than one row is returned I am in deep trouble and
I better consider shutting down both my application and the
database for serious investigation - rollback is in order
- if zero rows are returned my row was either deleted (eg.
nothing found for pk=1) or it was updated by someone else
(eg. xmin != <old_xmin>),
from this point on I am entering the slow path anyways (eg.
notifying the user, merge handling, delete detection etc.),
so rollback is in order, too
IOW I should be fine looking at xmin only for *detecting* a
concurrency conflict - be it concurrent updates or the row
having been deleted.
Am I getting this right ?
GPG key ID E4071346 @ wwwkeys.pgp.net
E167 67FD A291 2BEA 73BD 4537 78B9 A9F9 E407 1346
In response to
pgsql-novice by date
|Next:||From: Michael Guerin||Date: 2004-10-29 19:15:41|
|Subject: Postgresql crash- any ideas?|
|Previous:||From: Martijn van Oosterhout||Date: 2004-10-29 09:34:12|
|Subject: Re: Question Regarding Locks|
pgsql-general by date
|Next:||From: Michael Shtelma||Date: 2004-10-29 10:42:17|
|Subject: PostgreSQL on Windows|
|Previous:||From: Marco Colombo||Date: 2004-10-29 10:38:29|
|Subject: Re: Reasoning behind process instead of thread based|