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

From: Cédric Villemain <cedric(at)2ndquadrant(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Cc: Amit Kapila <amit(dot)kapila(at)huawei(dot)com>, "'Robert Haas'" <robertmhaas(at)gmail(dot)com>, "'Greg Smith'" <greg(at)2ndquadrant(dot)com>, "'Josh Berkus'" <josh(at)agliodbs(dot)com>, "'Tom Lane'" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "'Magnus Hagander'" <magnus(at)hagander(dot)net>, "'Christopher Browne'" <cbbrowne(at)gmail(dot)com>
Subject: Re: Proposal for Allow postgresql.conf values to be changed via SQL
Date: 2012-11-16 08:24:38
Message-ID: 201211160924.44112.cedric@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Le vendredi 16 novembre 2012 07:16:09, Amit Kapila a écrit :
> On Thursday, November 15, 2012 11:28 PM Cédric Villemain wrote:
> > Le jeudi 15 novembre 2012 15:48:14, Amit kapila a écrit :
> > > On Wednesday, November 14, 2012 12:24 AM Robert Haas wrote:
> > > 
> > > On Mon, Nov 12, 2012 at 10:59 PM, Amit kapila <amit(dot)kapila(at)huawei(dot)com>
> > 
> > wrote:
> > > > Uh, no, I don't think that's a good idea.  IMHO, what we should do
> > 
> > is:
> > > > 1. Read postgresql.conf.auto and remember all the settings we saw.
> > > > If we see something funky like an include directive, barf. 2. Forget
> > > > the value we remembered for the particular setting being changed.
> > > > Instead, remember the user-supplied new value for that parameter. 3.
> > > > Write a new postgresql.conf.auto based on the information remembered
> > > > in steps 1 and 2.
> > > 
> > > Attached patch contains implementaion for above concept.
> > > It can be changed to adapt the write file based on GUC variables as
> > > described by me yesterday or in some other better way.
> > > 
> > > Currenty to remember and forget, I have used below concept:
> > > 1. Read postgresql.auto.conf in memory.
> > > 2. parse the GUC_file for exact loction of changed variable 3. update
> > > the changed variable in memory at location found in step-2 4. Write
> > > the postgresql.auto.conf
> > > 
> > > Overall changes:
> > > 1. include dir in postgresql.conf at time of initdb 2. new built-in
> > > function pg_change_conf to change configuration settings 3. write file
> > > as per logic described above.
> > > 
> > > Some more things left are:
> > > 1. Transactional capability to command, so that rename of .lock file
> > > to .auto.conf can be done at commit time.
> > > 
> > > I am planing to upload the attached for this CF.
> > > 
> > > Suggestions/Comments?
> > 
> > Yes, sorry to jump here so late.
> > * Why don't we use pg_settings ? (i.e. why not materialize the view and
> > use it, it should be easier to have something transactional and also
> > serializable with probably a DEFERRABLE select pg_reload_config() which
> > mv the configuration file at commit time)
> 
> I think some consistency issues can come, because before editing and
> flushing, each backend has to have latest copy
> else it might override some parameters values.
> Can you explain the whole idea in detail, may be it will be easier to
> verify if it has any problem.

It looks like a bit similar to what proposed Peter in another thread.
If you use a table to store the values, the action of writing the file is just 
flush a table to disk, it might be a deferred trigger for example.
This table can be inserted/updated/deleted in a « BEGIN TRANSACTION ISOLATION 
SERIALIZABLE » transaction so there is no issue on who touch what and when. 
Either it is commited without serialization error or it is not. 
(and we can elaborate with the table content being deleted at commit time, or 
not, etc.).

I suppose it can be an extension also.

> 
> > * Can I define automatic parameters to be loaded before and/or after the
> > non- automatic parameters in a convenient way (without editing files at
> > all)?
> 
> In the current implementation, there is no way. Do you have any suggestion?

not yet.
...thinking some more...
Maybe add a column to define where to write the updated GUC (before everything 
else, after everything else, instead of the current content). The trigger 
responsible to write that will do.
Currently, nothing prevent 2 users to log in system, edit postgresql.conf and 
both issue a reload... So what's the problem ? Only have a single new GUC like 
'conf_from_sql=on|off' so it is possible to forbid update of configuration from 
SQL (it looks like a requirement for this new feature, anyway); or have it 
like an extension, and superuser are free to install or not.

Maybe I am repeating some previous suggestions which have been invalidated. In 
such case please excuse that I did not take time on my side to re-read all 
relative threads.
-- 
Cédric Villemain +33 (0)6 20 30 22 52
http://2ndQuadrant.fr/
PostgreSQL: Support 24x7 - Développement, Expertise et Formation

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