From: | Craig Ringer <craig(at)2ndquadrant(dot)com> |
---|---|
To: | Andres Freund <andres(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Reporting the commit LSN at commit time |
Date: | 2014-08-10 00:50:58 |
Message-ID: | 53E6C1F2.2050801@2ndquadrant.com |
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Lists: | pgsql-hackers |
On 08/10/2014 12:54 AM, Andres Freund wrote:
> On 2014-08-07 21:02:54 -0400, Tom Lane wrote:
>> Craig Ringer <craig(at)2ndquadrant(dot)com> writes:
>>> On 08/08/2014 03:54 AM, Tom Lane wrote:
>>>> FWIW, I think it's a seriously bad idea to expose LSNs in the protocol
>>>> at all. What happens five years from now when we switch to some other
>>>> implementation that doesn't have LSNs?
>>
>>> Everyone who's relying on them already via pg_stat_replication, etc, breaks.
>>> They're _already_ exposed to users. That ship has sailed.
>>
>> They're exposed to replication tools, yeah, but embedding them in the
>> wire protocol would be moving the goalposts a long way past that. As an
>> example of something that doubtless seemed like a good idea at the time,
>> consider the business about how an INSERT command completion tag includes
>> the OID of the inserted row. We're stuck with that obsolete idea
>> *forever* because it's embedded in the protocol for all clients.
>
> I don't think we really need to embed it at that level. And it doesn't
> have to be always on - only clients that ask for it need to get the
> answer. Something like COMMIT WITH (report_commit_lsn ON); or similar
> might do the trick?
Wouldn't that force client drivers - libpq, psqlODBC, PgJDBC, etc - to
all watch for explicit "COMMIT"s sent by the application and rewrite them?
Applications could also then request the commit option via a driver that
couldn't cope with it - which I think was one of Tom's concerns re using
a GUC, too.
--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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