Re: what happens when you issue ALTER SERVER in a hot environment?

"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Monday, April 6, 2020, AC Gomez <antklc(at)gmail(dot)com> wrote:
>> If you issue an ALTER SERVER command and there are active connections
>> with that server in use or new ones are coming in, what happens? Docs on
>> this command say nothing regarding active processing using the server
>> context and changes to it. So I assume it's just handled.

> Not sure if there are exceptions but assume that nothing external will
> change your current active session’s settings out from underneath you.

Couple of comments here:

* ALTER SERVER changes nothing until "pg_ctl reload" or similar is
issued. The same goes for manually editing the config file (which
is more or less the same thing, though on a different file).

* Once you do issue a reload, individual sessions will adopt the
new setting reasonably promptly, though not necessarily all at the
same instant (assuming it's a setting that doesn't require a
postmaster restart to take effect, in which case nothing happens).

* A session that has a higher-priority source for the setting of
a given GUC, such as a locally-issued SET, is not going to adopt
the new setting from the config file. But it *will* adopt that
setting on-the-fly if it has no other source of the setting.
The priority rules are explained in the fine manual, IIRC.
Possibly some clarification is needed?

>> For example if you alter user/password, I assume that as long as prior
>> user password is still good that actively running processes will keep going.

> If you alter a password the old one is by definition no longer good...but
> it doesn’t matter because authentication only happens once - during login.

Right. Also, passwords are *not* managed through the GUC (server
parameter) mechanism; what I just said about GUCs doesn't apply
to them.

regards, tom lane