Re: ALTER SYSTEM SET command to change postgresql.conf parameters (RE: Proposal for Allow postgresql.conf values to be changed via SQL [review])

On Thu, Aug 22, 2013 at 6:06 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> * Amit Kapila (amit(dot)kapila16(at)gmail(dot)com) wrote:
>> This can resolve the problem of whether to read auto file rather
>> cleanly, so the idea is:
>>
>> Enable/Disable reading of auto file
>> -----------------------------------------------------
>> a. Have a new include in postresql.conf
>> #include_auto_conf_file postgresql.auto.conf
>> as it is a special include, we can read this file relative to data
>> directory.
>>
>> Enable/Disable Alter System command
>> -----------------------------------------------------------
>> This can be achieved in 3 ways:
>> a. Check before executing Alter System if include directive is
>> disabled, then just issue a warning to user and proceed with command.
>> b. Check before executing Alter System if include directive is
>> disabled, then just issue an error and stop.
>
> It doesn't make sense for it to be a 'warning' with this- the
> parameter specifies the file to use. If you don't know what file to
> use, how you can possibly do anything but return an error?

As the file and location are fixed, we can go-ahead and write to
it, but I think now we are deciding
if someone disables include dir, then we can just disable Alter
System, so it is better to return error in such
situation.

> Note that I *like* that about this approach.
>
> There are a few other considerations with this-
>
> - What should the default be? (Still thinking 'off' myself)
default 'off' is a safe option, as it won't allow users to make
any change to parameter values until/unless they
read from manual, how to use it and what can go wrong, on the
other side it will be bit hassle for user to use this
command. I think 'on' would be better.
> - What happens if the user specifies 'postgresql.conf'? I'm thinking we
> would disallow such insanity (as that's what it is, imv..) by having
> an identifier in the file that this is the PG "auto conf" file.
I think we can detect by name and give error.
> - Should we have such an identifier in auto.conf to indicate that we
> created it, to prevent the user from setting it to something they
> shouldn't?
I think if user plays with this file manually, it can lead to
problems, that's why earlier we have
decided to keep a note on top of file which will indicate, do not
edit this file manually.
I believe that should be sufficient.

> - What's the "bootstrap" mode; iow, if a user enables the option but the
> file doesn't exist, what do we do? With this approach, I'd be
> inclined to say we simply create it and put the marker to indicate
> it's "our" file.

Alter System will create the file if doesn't exist.

> - Should we allow it to be outside of the data dir? We could simply log
> an error and ignore the parameter if it's more than a simple filename.

This should be an error, the file location and name will be fixed.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com