Re: Possibly too stringent Assert() in b-tree code

On Mon, Sep 26, 2016 at 8:54 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Sun, Sep 25, 2016 at 9:31 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:
>>> On Fri, Sep 23, 2016 at 12:21 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>>> This is clearly an oversight in Simon's patch fafa374f2, which introduced
>>>> this code without any consideration for the possibility that the page
>>>> doesn't have a valid special area. ...
>>>> but I'm not very clear on whether this is a safe fix, mainly because
>>>> I don't understand what the reuse WAL record really accomplishes.
>>>> Maybe we need to instead generate a reuse record with some special
>>>> transaction ID indicating worst-case assumptions?
>>
>>> I think it is basically to ensure that the queries on standby should
>>> not be considering the page being recycled as valid and if there is
>>> any pending query to which this page is visible, cancel it. If this
>>> understanding is correct, then I think the fix you are proposing is
>>> correct.
>>
>> After thinking about it some more, I do not understand why we are emitting
>> WAL here at all in *any* case, either for a new page or for a deleted one
>> that we're about to recycle. Why wouldn't the appropriate actions have
>> been taken when the page was deleted, or even before that when its last
>> tuple was deleted?
>>
>
> It seems to me that we do take actions for conflict resolution during
> the page deletion (that looks to be covered by XLOG_HEAP2_CLEANUP_INFO
> which we emit in vacuum), but not sure if that is sufficient.
> Consider a case where the new transaction is started on standby after
>

Here by new transaction, I intend to say some newer snapshot with
valid MyPgXact->xmin.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com