Re: [HACKERS] [PATCH] Add two-arg for of current_setting(NAME,FALLBACK)

On Thu, Mar 19, 2015 at 05:41:02PM -0500, David Christensen wrote:
> The two-arg form of the current_setting() function will allow a
> fallback value to be returned instead of throwing an error when an
> unknown GUC is provided.  This would come in most useful when using
> custom GUCs; e.g.:

There already _is_ a two-argument form of current_setting() that yours
somewhat conflicts with:
  current_setting(setting_name [, missing_ok ])

https://www.postgresql.org/docs/current/static/functions-admin.html

I often use
 coalesce(current_setting(setting_name, true), default_value_here)

as an implementation of current_setting() with a default value.

You could treat booleans as the second argument as a missing_ok argument
instead of a default value, since _currently_ current_setting() only
returns TEXT.

But if ever GUCs are allowed to have values of other types, then your
two-argument current_setting() will conflict with boolean GUCs.

There are several ways to prevent such a future conflict.  Here are
some:
- make a two-argument current_setting() for boolean GUCs treat the  second argument as a default value (since there are
nosuch GUCs  today, this won't break anything)
 
- use a new function name
- use a three-argument current_setting()

The third option seems very lame to me.  So I'd argue for either of the
other two.

Nico
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