Re: [Proposal] Global temporary tables

Dear Hackers


I attached the patch of GTT implementationI base on PG12.
The GTT design came from my first email.
Some limitations in patch will be eliminated in later versions.

Later, I will comment on Konstantin's patch and make some proposals for cooperation.
Looking forward to your feedback.

Thanks.

Zeng Wenjing





> 2019年10月29日 上午12:40，Robert Haas <robertmhaas@gmail.com> 写道：
>
> On Mon, Oct 28, 2019 at 9:37 AM Konstantin Knizhnik
> <k.knizhnik@postgrespro.ru> wrote:
>> Sorry, but both statements are not true.
>
> Well, I think they are true.
>
>> I am not sure how vital is lack of aborts for transactions working with
>> GTT at replica.
>> Some people said that there is no sense in aborts of read-only
>> transactions at replica (despite to the fact that them are saving
>> intermediate results in GTT).
>> Some people said something similar with your's "dead on arrival".
>> But inconsistency is not possible: if such transaction is really
>> aborted, then backend is terminated and nobody can see this inconsistency.
>
> Aborting the current transaction is a very different thing from
> terminating the backend.
>
> Also, the idea that there is no sense in aborts of read-only
> transactions on a replica seems totally wrong. Suppose that you insert
> a row into the table and then you go to insert a row in each index,
> but one of the index inserts fails - duplicate key, out of memory
> error, I/O error, whatever. Now the table and the index are
> inconsistent. Normally, we're protected against this by MVCC, but if
> you use a solution that breaks MVCC by using the same XID for all
> transactions, then it can happen.
>
>> Concerning second alternative: you can check yourself that it is not
>> *extremely* complicated and invasive.
>> I extracted changes which are related with handling transactions at
>> replica and attached them to this mail.
>> It is just 500 lines (including diff contexts). Certainly there are some
>> limitation of this implementation: number of  transactions working with
>> GTT at replica is limited by 2^32
>> and since GTT tuples are not frozen, analog of GTT CLOG kept in memory
>> is never truncated.
>
> I admit that this patch is not lengthy, but there remains the question
> of whether it is correct. It's possible that the problem isn't as
> complicated as I think it is, but I do think there are quite a number
> of reasons why this patch wouldn't be considered acceptable...
>
>> I agree with it and think that implementation of GTT with private
>> buffers and no replica access is good starting point.
>
> ...but given that we seem to agree on this point, perhaps it isn't
> necessary to argue about those things right now.
>
> --
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company