Re: Schema variables - new implementation for Postgres 15

> On Wed, May 22, 2024 at 02:37:49PM +0200, Peter Eisentraut wrote:
> On 18.05.24 13:29, Alvaro Herrera wrote:
> > I want to note that when we discussed this patch series at the dev
> > meeting in FOSDEM, a sort-of conclusion was reached that we didn't want
> > schema variables at all because of the fact that creating a variable
> > would potentially change the meaning of queries by shadowing table
> > columns.  But this turns out to be incorrect: it's_variables_  that are
> > shadowed by table columns, not the other way around.
>
> But that's still bad, because seemingly unrelated schema changes can make
> variables appear and disappear.  For example, if you have
>
> SELECT a, b FROM table1
>
> and then you drop column b, maybe the above query continues to work because
> there is also a variable b.  Or maybe it now does different things because b
> is of a different type.  This all has the potential to be very confusing.

Yeah, that's a bummer. Interestingly enough, the db2 implementation of
global session variables mechanism is mentioned as similar to what we
have in the patch. But weirdly, the db2 documentation just states
possibility of a resolution conflict for unqualified names, nothing
else.