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 ])


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

 - make a two-argument current_setting() for boolean GUCs treat the
   second argument as a default value (since there are no such 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.


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