Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> Robert Haas wrote:
>> On Thu, Sep 17, 2009 at 10:21 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> Anything that moves tuples is not acceptable as a hidden background
>>> operation, because it will break applications that depend on CTID.
>> I'm a bit confused. CTIDs change all the time anyway, whenever you
>> update the table. What could someone possibly be using them for?
> As a unique identifier, while you hold a portal open.
Or for an update without having to hold a transaction open. We have
recommended this type of technique in the past:
select ctid, xmin, * from table where id = something;
... allow user to edit the row at his leisure ...
update table set ... where id = something and ctid = previous value and xmin = previous value;if rows_updated = 0
then report error ("row was already updated by someone else");
(Actually, the ctid is only being used for fast access here; the xmin
is what is really needed to detect that someone else updated the row.
But the proposed tuple-mover would break the xmin check too.)
> It's no different from the situation where another backend UPDATEs the
> row under your nose, but it's not something you want to do automatically
> without notice.
Exactly. The application is typically going to throw a "concurrent
update" type of error when this happens, and we don't want magic
background operations to cause that.
regards, tom lane