Re: Schema variables - new implementation for Postgres 15

Hi,

On Tue, Jan 18, 2022 at 10:01:01PM +0100, Pavel Stehule wrote:
> pá 14. 1. 2022 v 3:44 odesílatel Julien Rouhaud <rjuju123@gmail.com> napsal:
>
> >
> > =# let myvariable = 'AA';
> > LET
> >
> > =# select 'AA' collate "en-x-icu" < myvariable;
> >  ?column?
> > ----------
> >  f
> > (1 row)
> >
> > =# select 'AA' collate "en-x-icu" < myvariable collate mycollation;
> > ERROR:  42P21: collation mismatch between explicit collations "en-x-icu"
> > and "mycollation"
> > LINE 1: select 'AA' collate "en-x-icu" < myvariable collate mycollat...
> >
>
> What do you expect?  I don't understand collating well, but it looks
> correct. Minimally the tables have the same behavior.

Indeed, I actually didn't know that such object's collation were implicit and
could be overloaded without a problem as long as there's no conflict between
all the explicit collations.  So I agree that the current behavior is ok,
including a correct handling for wanted conflicts:

=# create variable var1 text collate "fr-x-icu";
CREATE VARIABLE

=# create variable var2 text collate "en-x-icu";
CREATE VARIABLE

=# let var1 = 'hoho';
LET

=# let var2 = 'hoho';
LET

=# select var1 < var2;
ERROR:  42P22: could not determine which collation to use for string comparison
HINT:  Use the COLLATE clause to set the collation explicitly.

> Please, can you check the attached patches?

All the issue I mentioned are fixed, thanks!

I see a few problems with the other new features added though.  The new
session_variables_ambiguity_warning GUC is called even in contexts where it
shouldn't apply.  For instance:

=# set session_variables_ambiguity_warning = 1;
SET

=# create variable v text;
CREATE VARIABLE

=# DO $$
DECLARE v text;
BEGIN
v := 'test';
RAISE NOTICE 'v: %', v;
END;
$$ LANGUAGE plpgsql;
WARNING:  42702: session variable "v" is shadowed by column
LINE 1: v := 'test'
        ^
DETAIL:  The identifier can be column reference or session variable reference.
HINT:  The column reference is preferred against session variable reference.
QUERY:  v := 'test'

But this "v := 'test'" shouldn't be a substitute for a LET, and it indeed
doesn't work:

=# DO $$
BEGIN
v := 'test';
RAISE NOTICE 'v: %', v;
END;
$$ LANGUAGE plpgsql;
ERROR:  42601: "v" is not a known variable
LINE 3: v := 'test';

But the RAISE NOTICE does see the session variable (which should be the correct
behavior I think), so the warning should have been raised for this instruction
(and in that case the message is incorrect, as it's not shadowing a column).