On 2/14/07, Tom Lane <tgl@sss.pgh.pa.us> wrote: Heikki Linnakangas <heikki@enterprisedb.com> writes:
> What's the verdict on relaxing the "live tuple's ctid doesn't change
> rule"?
I think that's unacceptable; it is known that that will break the ODBC
and JDBC drivers, as well as any other programs that make use of the
ctid for re-finding a tuple they read earlier in the same transaction.
We have not only never deprecated client-side use of ctid for this, but
actively encouraged it, for instance by going out of our way to support
fast access for queries "WHERE ctid = 'constant'".
What's more, your proposal would break plain old UPDATE and DELETE,
as well as SELECT FOR UPDATE, none of which promise to hold a pin
continuously on every page containing a tuple they might decide to
revisit (by ctid) later. Are you prepared to disallow hash join and
sort/merge join in all such queries?
Not that I am suggesting we do this, but I believe we had some solution
to this problem in the earlier version of HOT. The live tuple when copied-back
to the root tuple, the tuple is marked with a HEAP_COPIED_BACK flag.
HeapTupleSatisfiesUpdate() checks for this flag and if set returns a new
return code, HeapTupleCopiedBack. heap_update() returns the same
to ExecUpdate along with the ctid of the root tuple. The UPDATE/DELETE
operation then retried on the root tuple, very similar to read-committed
update/delete. The xmax of the copied-back tuple is set so that its
not vacuumed away until all the current transactions are completed.
Though I have tested this patch several times and it seems to work fine,
I probably don''t have insight into the code as much others on this list
has. So if someone wants to take a look and see if it would work fine,
I would be more than happy to post the latest HOT patch (older design).
Thanks,
Pavan
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