Re: Clarify "allow_system_table_mods"

On Mon, Apr 25, 2016 at 7:57 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:

> David, Melvin,
>
> * David G. Johnston (david(dot)g(dot)johnston(at)gmail(dot)com) wrote:
> > On Monday, April 25, 2016, Melvin Davidson <melvin6925(at)gmail(dot)com> wrote:
> > > I need clarification on allow_system_table_mods parameter
> > > Per the documentation:
> > > *Allows modification of the structure of system tables.* This is used
> by
> > > initdb. This parameter can only be set at server start.
> > >
> > > However, attempting to modify pg_class to add another column fails with
> > > "STATUS_ACCESS_VIOLATION" as below.
> > > So either only certain system catalogs may be changed, or only certain
> > > types of structure changes are allowed.
>
> There might be some things about system catalogs you can modify, but
> generally speaking, you can't add a column or otherwise change the
> structure. System catalogs are represented in memory by C structures
> (and more), so it's not too surprising that adding a column causes a
> crash.
>
> > If it's going to prevent the action it should be able to do so without
> > crashing the server..and I doubt it's intended to be opinionated aside
> from
> > covering all DDL. What little there is on Google seems to support this.
>
> Sorry, but as a superuser, there's a lot of things you can do to crash
> the server, this is just one way. The effort required to prevent
> anything bad from happening when a user is running as a superuser is far
> from trivial.
>
> In short, no, you can't just add a column to pg_class via SQL, and I
> don't think we're going to be very interested in trying to "fix" such
> cases.
>
> Thanks!
>
> Stephen
>

Thank you Stephen.
Then could you please clarify exactly what structural mods are permitted by
*"*
*Allows modification of the structure of system tables" ?*

--
*Melvin Davidson*
I reserve the right to fantasize. Whether or not you
wish to share my fantasy is entirely up to you.