Re: user defined settings (aka user defined guc variables)

What exactly can you do with these variables other than SHOW/SET.  Seems
it would be nice if they could be used in queries, like in a special
table like sysvar:
SELECT sysvar.fsync;

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Joe Conway wrote:
> I've been playing around with making it possible to create user defined guc 
> variables. This has been discussed, at least in passing, before. And it is 
> even anticipated in guc.c as a possible future feature:
> /*
>   * Build the sorted array.    This is split out so that it could be
>   * re-executed after startup (eg, we could allow loadable modules to
>   * add vars, and then we'd need to re-sort).
>   */
> 
> It is a feature that would be nice to have, so that, for example, a user 
> defined variable named "my_classpath" could be created to point to the java 
> CLASSPATH needed by a custom C function.
> 
> So far I have this much working:
> - A new backend function, pg_create_user_setting(name, value, islocal) is used
>    to "register" the setting.
> - SHOW ALL, SHOW, current_setting(), and pg_show_all_settings()) will display
>    it just like any other setting
> - Similarly, SET and set_config() will change it.
> 
> I still need to make the user defined settings survive being saved by ALTER 
> USER or ALTER DATABASE. I'm also thinking about a corresponding grammar 
> addition, something along the lines of:
> 
>    CREATE SETTING name WITH VALUE value;
> 
> This would effectively perform:
>    SELECT pg_create_user_setting(name, value, false);
> 
> I'm wondering whether it would be "a good thing" or "a bad thing" to have 
> unrecognized settings found in postgresql.conf be registered as user defined 
> settings?
> 
> Any comments, concerns, or objections?
> 
> Thanks,
> 
> Joe
> 
> 
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