Re: Reviewing freeze map code

On Tue, Jun 28, 2016 at 8:06 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> On Tue, Jun 21, 2016 at 6:59 AM, Andres Freund <andres@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 <andres@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).
>

I understood now, sorry for the noise.

Regards,

--
Masahiko Sawada