Date: Fri, 14 Jan 2022 10:44:02 +0800
From: Julien Rouhaud <rjuju123@gmail.com>
To: Pavel Stehule <pavel.stehule@gmail.com>
Cc: Dean Rasheed <dean.a.rasheed@gmail.com>,
	Joel Jacobson <joel@compiler.org>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Schema variables - new implementation for Postgres 15
Message-ID: <20220114024402.btyn7vrpkhnso2bq@jrouhaud>
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Hi,

On Thu, Jan 13, 2022 at 07:32:26PM +0100, Pavel Stehule wrote:
> čt 13. 1. 2022 v 19:23 odesílatel Dean Rasheed <dean.a.rasheed@gmail.com>
> napsal:
> 
> > On Thu, 13 Jan 2022 at 17:42, Pavel Stehule <pavel.stehule@gmail.com>
> > wrote:
> > >
> > > I like the idea of prioritizing tables over variables with warnings when
> > collision is detected. It cannot break anything. And it allows to using
> > short identifiers when there is not collision.
> >
> > Yeah, that seems OK, as long as it's clearly documented. I don't think
> > a warning is necessary.

What should be the behavior for a cached plan that uses a variable when a
conflicting relation is later created?  I think that it should be the same as a
search_path change and the plan should be discarded.

> The warning can be disabled by default, but I think it should be there.
> This is a signal, so some in the database schema should be renamed. Maybe -
> session_variables_ambiguity_warning.

I agree that having a way to know that a variable has been bypassed can be
useful.

> > (FWIW, testing with dbfiddle, that appears to match Db2's behaviour).
> >
> 
> Thank you for check

Do you know what's oracle's behavior on that?


I've been looking at the various dependency handling, and I noticed that
collation are ignored, while they're accepted syntax-wise:

=# create collation mycollation (locale = 'fr-FR', provider = 'icu');
CREATE COLLATION

=# create variable myvariable text collate mycollation;
CREATE VARIABLE

=# select classid::regclass, objid, objsubid, refclassid::regclass, refobjid, refobjsubid from pg_depend where classid::regclass::text = 'pg_variable' or refclassid::regclass::text = 'pg_variable';
   classid   | objid | objsubid |  refclassid  | refobjid | refobjsubid
-------------+-------+----------+--------------+----------+-------------
 pg_variable | 16407 |        0 | pg_namespace |     2200 |           0
(1 row)

=# 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...

So it's missing both dependency recording for variable's collation and also
teaching various code that variables can have a collation.

It's also missing some invalidation detection.  For instance:

=# create variable myval text;
CREATE VARIABLE

=# let myval = 'pg_class';
LET

=# prepare s(text) as select relname from pg_class where relname = $1 or relname = myval;
PREPARE

=# set plan_cache_mode = force_generic_plan ;
SET

=# execute s ('');
 relname
----------
 pg_class
(1 row)

=# drop variable myval ;
DROP VARIABLE

=# create variable myval int;
CREATE VARIABLE

=# execute s ('');
ERROR:  XX000: cache lookup failed for session variable 16408

The plan should have been discarded and the new plan should fail for type
problem.

Strangely, subsequent calls don't error out:

=# execute s('');
 relname
---------
(0 rows)

But doing an explain shows that there's a problem:

=# explain execute s('');
ERROR:  XX000: cache lookup failed for variable 16408