Re: [HACKERS] Re: Is anything preventing us from allowing write toforeign tables from standby?

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От Alexander Korotkov
Тема Re: [HACKERS] Re: Is anything preventing us from allowing write toforeign tables from standby?
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Msg-id CAPpHfdsgD6-EED-P8TuAf4OFqXnT16xuHdQ8Je9NUKfYjRj0jA@mail.gmail.com
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Ответ на Re: [HACKERS] Re: Is anything preventing us from allowing write toforeign tables from standby?  (Michael Paquier <michael.paquier@gmail.com>)
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On Wed, Nov 29, 2017 at 5:07 AM, Michael Paquier <michael.paquier@gmail.com> wrote:
On Fri, Oct 27, 2017 at 4:44 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> However, as Michael also points out, it's arguably wrong to allow a
> nominally read-only transaction to write data regardless of whether it
> works.  In the case of a standby it could be argued that your
> transaction is only read-only because you had no other choice, but
> nonetheless that's how it is marked.  I have a feeling that if we
> extend the definition of "read only" to mean "sometimes allow writes",
> we may regret it.

I still have the same feeling. What I am sure of is that this patch is
not the correct way to do things. So I am marking it as returned with
feedback. This is not a rejection from my side, as I think that this
feature could be useful in some cases, but its design needs way more
thoughts.

Righ, my initial proposal was naive.  This discussion raised at least two design issues.

1) It doesn't seem OK from the design point of view to allow read-only transaction to write, even if it's write to foreign server.  One possible solution is to distinguish three transaction types: real-only, write only to foreign servers, read-write.
2) We have upcoming patch implementing atomic commit to multiple foreign servers.  This atomic commit feature wouldn't work on standby, since it writes to local tables.  We should detect foreign server need in real read-write transaction for writing to foreign server, and prevent such writes on standby.

I'll try to propose patch addressing these two issues to the next commitfest.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company 

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