Re: Unexpected behaviour: it was documented to return the same value

On Saturday, July 5, 2025, PG Doc comments form <noreply(at)postgresql(dot)org>
wrote:

> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/17/functions-admin.html
> Description:
>
> set_config ( setting_name text, new_value text, is_local boolean ) → text
> Sets the parameter setting_name to new_value, and returns that value. If
> is_local is true, the new value will only apply during the current
> transaction. If you want the new value to apply for the rest of the current
> session, use false instead. This function corresponds to the SQL command
> SET.
> set_config('log_statement_stats', 'off', false) → off
> ```
> invoice=> select set_config( 'app.asdf', null, false ) is null;
> ?column?
> ----------
> f
> (1 row)
> invoice=> select set_config( 'app.asdf', null, false ) = '';
> ?column?
> ----------
> t
> (1 row)
> ```
> Expected: because I provided NULL value I expect to receive NULL as result
> Actual: but function returns the empty string instead.
> It would be better to follow documented behaviour, if it will not be
> possible to fix it, please at least document that.

There are pending patch(es?) in this area, I see to what extent this
specific wording is covered (not sure we are back-patching the new wording
either…). The main point to remember is that a configuration setting can
never take on the null value. All customized setting placeholders *that
exist* are text typed with a non-null value defaulting to the empty string.

David J.