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

> On Tuesday, July 23, 2013 5:26 AM Tom Lane wrote:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
> > Christophe just discovered something with include files which is
> going
> > to cause issues with ALTER SYSTEM SET.
>
> > So, take as a hypothetical that you use the default postgresql.conf
> > file, which sets shared_buffers = 32MB.
>
> > Instead of editing this file, you do ALTER SYSTEM SET shared_buffers
> =
> > '1GB', which updates config/postgresql.auto.conf.
>
> > Then you restart PostgreSQL.
>
> > Everything is hunky-dory, until a later occasion where you *reload*
> > postgresql.
>
> Everything was already *not* hunky-dory as soon as you did that, since
> a SIGHUP would have had the same problem.
>
> I'd be inclined to think that ALTER SYSTEM SET should not be allowed to
> modify any PGC_POSTMASTER parameters.

One way to fix the problem is that while parsing if the option already
exists, replace it.
Something like below code

ParseConfigFp()
{
..
..
/* replace the option value, if already exists in list */
for (item = *head_p; item != NULL; item =
item->next)
{
if (strcmp(item->name, opt_name) == 0)
{
pfree(item->value);
item->value = pstrdup(opt_value);
replaced = true;
break;
}
}
if(!replaced)
{
/* ordinary variable, append to list */
item = palloc(sizeof *item);
item->name = opt_name;
item->value = opt_value;
item->filename = pstrdup(config_file);
item->sourceline = ConfigFileLineno-1;
item->next = NULL;
if (*head_p == NULL)
*head_p = item;
else
(*tail_p)->next = item;
*tail_p = item;
}

..
..
}

There is overhead of traversing the list each time, but as this path is
traversed in less and non-performance critical operations, it can be
considered to fix the problem.

If you consider above as a non-trivial or not a right way to fix the
problem,
then I can update the patch to disallow PGC_POSTMASTER parameters by ALTER
SYSTEM SET command.

With Regards,
Amit Kapila.