Subject: Re: [PATCH] Stop ALTER SYSTEM from making bad assumptions
To: Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>
Cc: Stephen Frost <sfrost@snowman.net>, Amit Kapila
 <amit.kapila16@gmail.com>, Robert Haas <robertmhaas@gmail.com>,
 Tomas Vondra <tomas.vondra@2ndquadrant.com>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
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From: Ian Barwick <ian.barwick@2ndquadrant.com>
Message-ID: <e61fca0f-7ea7-d8bf-6230-a35e8f9a0911@2ndquadrant.com>
Date: Sat, 3 Aug 2019 08:22:29 +0900
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On 8/3/19 7:56 AM, Andres Freund wrote:
> Hi,
> 
> On 2019-08-02 18:47:07 -0400, Tom Lane wrote:
>> Stephen Frost <sfrost@snowman.net> writes:
>>> I disagree that this should only be addressed in alter system, as I’ve said
>>> before and as others have agreed with.  Having one set of code that can be
>>> used to update parameters in the auto.conf and then have that be used by
>>> pg_basebackup, alter system, and external tools, is the right approach.
>>
>> I don't find that to be necessary or even desirable.  Many (most?) of the
>> situations where this would be important wouldn't have access to a running
>> backend, and maybe not to any PG code at all --- what if your tool isn't
>> written in C?
> 
> I think a commandline tool to perform the equivalent of ALTER SYSTEM on
> a shutdown cluster would be a great tool. It's easy enough to add
> something with broken syntax, and further down the road such a tool
> could not only ensure the syntax is correct, but also validate
> individual settings as much as possible (obviously there's some hairy
> issues here).

What I came up with shoehorned a stripped-down version of the backend
config parser into fe_utils and provides a function to modify pg.auto.conf
in much the same way ALTER SYSTEM does, but with only the basic syntax
checking provided by the parser of course. And for completeness a
client utility which can be called by scripts etc.

I can clean it up and submit it later for reference (got distracted by other things
recently) though I don't think it's a particularly good solution due to the
lack of actual checks for the provided GUCSs (and the implementation
is ugly anyway); something like what Andres suggests below would be far better.

> Quite possibly the most realistic way to implement something like that
> would be a postgres commandline switch, which'd start up far enough to
> perform GUC checks and execute AlterSystem(), and then shut down
> again. We already have -C, I think such an option could reasonably be
> implemented alongside it.
> 
> Obviously this is widely out of scope for v12.


Regards


Ian Barwick


-- 
  Ian Barwick                   https://www.2ndQuadrant.com/
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