Re: Schema variables - new implementation for Postgres 15

> On Wed, Aug 23, 2023 at 04:02:44PM +0200, Pavel Stehule wrote:
> NameListToString is already buildin function. Do you think NamesFromList?
>
> This is my oversight - there is just `+extern List *NamesFromList(List
> *names); ` line, but sure - it should be in 0002 patch
>
> fixed now

Right, thanks for fixing.

I think there is a wrinkle with pg_session_variables function. It
returns nothing if sessionvars hash table is empty, which has two
consequences:

* One might get confused about whether a variable is created,
based on the information from the function. An expected behaviour, but
could be considered a bad UX.

=# CREATE VARIABLE var1 AS varchar;

-- empty, is expected
=# SELECT name, typname, can_select, can_update FROM pg_session_variables();
name | typname | can_select | can_update
------+---------+------------+------------
(0 rows)

-- but one can't create a variable
=# CREATE VARIABLE var1 AS varchar;
ERROR: 42710: session variable "var1" already exists
LOCATION: create_variable, pg_variable.c:102

-- yet, suddenly after a select...
=# SELECT var2;
var2
------
NULL
(1 row)

-- ... it's not empty
=# SELECT name, typname, can_select, can_update FROM pg_sessio
n_variables();
name | typname | can_select | can_update
------+-------------------+------------+------------
var2 | character varying | t | t
(1 row)

* Running a parallel query will end up returning an empty result even
after accessing the variable.

-- debug_parallel_query = 1 all the time
=# CREATE VARIABLE var2 AS varchar;

-- empty, is expected
=# SELECT name, typname, can_select, can_update FROM pg_session_variables();
name | typname | can_select | can_update
------+---------+------------+------------
(0 rows)

-- but this time an access...
SELECT var2;
var2
------
NULL
(1 row)

-- or set...
=# LET var2 = 'test';

-- doesn't change the result, it's still empty
=# SELECT name, typname, can_select, can_update FROM pg_session_variables();
name | typname | can_select | can_update
------+---------+------------+------------
(0 rows)

Would it be a problem to make pg_session_variables inspect the catalog
or something similar if needed?