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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