Re: Re: Proposal for Allow postgresql.conf values to be changed via SQL [review]

2013-03-13 07:42 keltezéssel, Craig Ringer írta:
> On 03/12/2013 06:27 AM, Craig Ringer wrote:
>>>> Think also about the case where someone wants to change multiple
>>>> values together and having just some set and not others would be
>>>> inconsistent.
>> Yeah, that's a killer. The reload would need to be scheduled for COMMIT
>> time, it can't be done by `SET PERSISTENT` directly.
> Thinking about this some more, I'm not sure this is a good idea.
>
> Right now, SET takes effect immediately. Always, without exception.
> Delaying SET PERSISTENT's effects until commit would make it
> inconsistent with SET's normal behaviour.
>
> However, *not* delaying it would make it another quirky
> not-transactional not-atomic command. That's OK, but if it's not going
> to follow transactional semantics it should not be allowed to run within
> a transaction, like VACUUM .
>
> Writing the changes immediately but deferring the reload until commit
> seems to be the worst of those two worlds.

I was thinking about it a little. There is a hook that runs at the end
of (sub-)transactions. It can be abused for this purpose to make
SET PERSISTENT transactional. The subtransactions can also stack
these settings, by forgetting settings upon ROLLBACK [ TO SAVEPOINT ]
and overwriting previous settings upon COMMIT and RELEASE SAVEPOINT.
All it needs is a list and maintaining intermediate pointers when entering
into a new level of SAVEPOINT. The functions to register such hooks are
in src/include/access/xact.h:

extern void RegisterXactCallback(XactCallback callback, void *arg);
extern void UnregisterXactCallback(XactCallback callback, void *arg);
extern void RegisterSubXactCallback(SubXactCallback callback, void *arg);
extern void UnregisterSubXactCallback(SubXactCallback callback, void *arg);

The per-session local effect can also be immediate, it just needs
the normal SET and SET PERSISTENT code paths to be unified.

On the other hand, it would make a lot of sense to implement it
as one setting per file or extending the code to allow modifying
settings in bulk. The one setting per file should be easier and
it would also allow extensions to drop their settings automatically
into the automatic config directory. I don't know who mentioned
this idea about extensions but it also came up a while back.

Best regards,
Zoltán Böszörményi

--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt, Austria
Web: http://www.postgresql-support.de
http://www.postgresql.at/