Re: BUG #15646: Inconsistent behavior for current_setting/set_config

On 2/20/19 12:11 PM, Tom Lane wrote:
> Joe Conway <mail(at)joeconway(dot)com> writes:
>> On 2/20/19 11:10 AM, PG Bug reporting form wrote:
>>> But current behavior returns empty string instead of NULL (the initial
>>> value) after transaction is rolled back. When I restart session, NULL is
>>> returned again as it is expected.
>
>> This has been discussed before and dismissed:
>> https://www.postgresql.org/message-id/flat/56842412.5000005%40joeconway.com
>> Personally I agree it is a bug, but I am not sure you will get much
>> support for that position.
>
> The fact that we allow undeclared user-defined GUCs at all is a bug IMO.
> We need to find a way to replace that behavior with something whereby
> the name and type of a parameter are declared up-front before you can
> set it.

(moving to hackers)

Perhaps we could do something like:

1. If the user-defined GUC is defined in postgresql.conf, et al, same
behavior as now
2. Backward compatibility concerns would be an issue, so create another
new GUC declare_custom_settings which initially defaults to false.
3. If declare_custom_settings is true, and the user-defined GUC is not
defined in postgresql.conf, then in order to create it dynamically
via SET or similar methods, you must do something like:

CREATE SETTING name TYPE guctype [LIST];
SET name TO value;

Joe

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