Re: proposal: schema variables

st 24. 7. 2024 v 19:02 odesílatel Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
napsal:

> On Wed, 2024-07-24 at 17:19 +0200, Pavel Stehule wrote:
> > > This is buggy:
> > >
> > > CREATE VARIABLE str AS text NOT NULL DEFAULT NULL;
> > >
> > > Ugh.
> > >
> > > SELECT str;
> > > ERROR: null value is not allowed for NOT NULL session variable
> "laurenz.str"
> > > DETAIL: The result of DEFAULT expression is NULL.
> > >
> > > Perhaps that is a leftover from the previous coding, but I think
> there need be
> > > no check upon SELECT. It should be enough to check during CREATE
> VARIABLE and
> > > LET.
> >
> > I think it is correct. When you use SELECT str, then DEFAULT expression
> is
> > executed, and then the result is assigned to a variable, and there is
> NOT NULL
> > guard, which raises an exception. The variable is not initialized when
> you run
> > DDL, but it is initialized when you first read or write from/to the
> variable.
> > The DEFAULT expression is not evaluated in DDL time. In this case, I can
> detect
> > the problem in DDL time because the result is transformed to NULL node,
> but
> > generally there can be SQL non immutable function, and then I need to
> wait until
> > the DEFAULT expression will be evaluated - and it is time of first
> reading.
> > Unfortunately, there is not an available check if some expression is
> NULL,
> > that I can use in DDL time, but I have it in plpgsql_check.
>
> That makes sense to me.
>
> In that case, I think we can drop the requirement that NOT NULL variables
> need a default clause.
>

removed

>
> >
> > > I can see the usefulness of IMMUTABLE variables, but I am surprised
> that
> > > they are reset by DISCARD. What is the use case you have in mind?
> > > The use case I can envision is an application that sets a value
> right after
> > > authentication, for use with row-level security. But then it would
> be harmful
> > > if the user could reset the variable with DISCARD.
> >
> > Primary I think about IMMUTABLE variables like about some form of cache.
> > This cache is protected against unwanted repeated write - and can help
> with
> > detection of this situation.
>
> We can leave it as it is. The IMMUTABLE feature need not go into the first
> release, so that can be discussed some more later.
>
> Thanks for the new patch set; I'll look at it soon.
>

Thank you very much

Regards

Pavel

>
> Yours,
> Laurenz Albe
>