On Tue, Aug 3, 2021 at 4:25 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Tue, Jul 27, 2021 at 9:56 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> > Yeah, that's a big problem, seems like the expression evaluation
> > machinery directly going and detoasting the externally stored data
> > using some random snapshot. Ideally, in walsender we can never
> > attempt to detoast the data because there is no guarantee that those
> > data are preserved. Somehow before going to the expression evaluation
> > machinery, I think we will have to deform that tuple and need to do
> > something for the externally stored data otherwise it will be very
> > difficult to control that inside the expression evaluation.
> >
>
> True, I think it would be possible after we fix the issue reported in
> another thread [1] where we will log the key values as part of
> old_tuple_key for toast tuples even if they are not changed. We can
> have a restriction that in the WHERE clause that user can specify only
> Key columns for Updates similar to Deletes. Then, we have the data
> required for filter columns basically if the toasted key values are
> changed, then they will be anyway part of the old and new tuple and if
> they are not changed then they will be part of the old tuple.
Right.
I have
> not checked the implementation part of it but theoretically, it seems
> possible.
Yeah, It would be possible to because at least after fixing [1] we
would have the required column data. The only thing I am worried
about is while applying the filter on the new tuple the toasted
unchanged key data will not be a part of the new tuple. So we can not
directly call the expression evaluation machinary, basically, somehow
we need to deform the new tuple and then replace the data from the old
tuple before passing it to expression evaluation. Anyways this is an
implementation part so we can look into that while implementing.
If my understanding is correct then it becomes necessary to
> solve the other bug [1] to solve this part of the problem for this
> patch.
Right.
The other possibility is to disallow columns (datatypes) that
> can lead to toasted data (at least for Updates) which doesn't sound
> like a good idea to me.
Yeah, that will be a big limitation, then we won't be able to allow
expression on any varlena types.
Do you have any other ideas for this problem?
As of now no other better idea to suggest.
[1] -
https://www.postgresql.org/message-id/OS0PR01MB611342D0A92D4F4BF26C0F47FB229%40OS0PR01MB6113.jpnprd01.prod.outlook.com
--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com
