On Tue, Jun 21, 2016 at 6:59 AM, Andres Freund <and...@anarazel.de> wrote:
> On 2016-06-20 17:55:19 -0400, Robert Haas wrote:
>> On Mon, Jun 20, 2016 at 4:24 PM, Andres Freund <and...@anarazel.de> wrote:
>> > On 2016-06-20 16:10:23 -0400, Robert Haas wrote:
>> >> What exactly is the point of all of that already_marked stuff?
>> >
>> > Preventing the old tuple from being locked/updated by another backend,
>> > while unlocking the buffer.
>> >
>> >> I
>> >> mean, suppose we just don't do any of that before we go off to do
>> >> toast_insert_or_update and RelationGetBufferForTuple.  Eventually,
>> >> when we reacquire the page lock, we might find that somebody else has
>> >> already updated the tuple, but couldn't that be handled by
>> >> (approximately) looping back up to l2 just as we do in several other
>> >> cases?
>> >
>> > We'd potentially have to undo a fair amount more work: the toasted data
>> > would have to be deleted and such, just to retry. Which isn't going to
>> > super easy, because all of it will be happening with the current cid (we
>> > can't just increase CommandCounterIncrement() for correctness reasons).
>> Why would we have to delete the TOAST data?  AFAIUI, the tuple points
>> to the TOAST data, but not the other way around.  So if we change our
>> mind about where to put the tuple, I don't think that requires
>> re-TOASTing.
> Consider what happens if we, after restarting at l2, notice that we
> can't actually insert, but return in the !HeapTupleMayBeUpdated
> branch. If the caller doesn't error out - and there's certainly callers
> doing that - we'd "leak" a toasted datum.

Sorry for interrupt you, but I have a question about this case.
Is there case where we back to l2 after we created toasted
datum(called toast_insert_or_update)?
IIUC, after we stored toast datum we just insert heap tuple and log
WAL (or error out for some reasons).


Masahiko Sawada

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