Re: [Proposal] Global temporary tables

On Mon, Oct 28, 2019 at 9:37 AM Konstantin Knizhnik
<k(dot)knizhnik(at)postgrespro(dot)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
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