On Tue, Jun 6, 2017 at 11:54 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Jun 5, 2017 at 2:51 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>>> Greg/Amit's idea of using the CTID field rather than an infomask bit
>>> seems like a possibly promising approach. Not everything that needs
>>> bit-space can use the CTID field, so using it is a little less likely
>>> to conflict with something else we want to do in the future than using
>>> a precious infomask bit. However, I'm worried about this:
>>>
>>> /* Make sure there is no forward chain link in t_ctid */
>>> tp.t_data->t_ctid = tp.t_self;
>>>
>>> The comment does not say *why* we need to make sure that there is no
>>> forward chain link, but it implies that some code somewhere in the
>>> system does or at one time did depend on no forward link existing.
>>
>> I think it is to ensure that EvalPlanQual mechanism gets invoked in
>> the right case. The visibility routine will return HeapTupleUpdated
>> both when the tuple is deleted or updated (updated - has a newer
>> version of the tuple), so we use ctid to decide if we need to follow
>> the tuple chain for a newer version of the tuple.
>
> That would explain why need to make sure that there *is* a forward
> chain link in t_ctid for an update, but it doesn't explain why we need
> to make sure that there *isn't* a forward link for delete.
>
As far as I understand, it is to ensure that for deleted rows, nothing
more needs to be done. For example, see the below check in
ExecUpdate/ExecDelete.
if (!ItemPointerEquals(tupleid, &hufd.ctid))
{
..
}
..
Also a similar check in ExecLockRows. Now for deleted rows, if the
t_ctid wouldn't point to itself, then in the mentioned functions, we
were not in a position to conclude that the row is deleted.
--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com