Re: proposal: schema variables

st 13. 11. 2024 v 17:35 odesílatel Dmitry Dolgov <9erthalion6@gmail.com>
napsal:

> > On Sun, Nov 10, 2024 at 06:51:40PM GMT, Pavel Stehule wrote:
> > ne 10. 11. 2024 v 17:19 odesílatel Pavel Stehule <
> pavel.stehule@gmail.com>
> > napsal:
> > I thought a lot of time about better solutions for identifier collisions
> > and I really don't think so there is some consistent user friendly
> syntax.
> > Personally I think there is an easy already implemented solution -
> > convention - just use a dedicated schema for variables and this schema
> > should not be in the search path. Or use secondary convention - like
> using
> > prefix "__" for session variables. Common convention is using "_" for
> > PLpgSQL variables. I searched how this issue is solved in other
> databases,
> > or in standard, and I found nothing special. The Oracle and SQL/PSM has a
> > concept of visibility - the variables are not visible outside packages or
> > modules, but Postgres has nothing similar. It can be emulated by a
> > dedicated schema without inserting a search path, but it is less strong.
> >
> > I think we can introduce an alternative syntax, that will not be user
> > friendly or readable friendly, but it can be without collisions - or can
> > decrease possible risks.
> >
> > It is nothing new - SQL does it with old, "new" syntax of inner joins, or
> > in Postgres we can
> >
> > where salary < 40000
> >
> > or
> >
> > where pg_catalog.int4lt(salary, 40000);
> >
> >
> > or some like we use for operators OPERATOR(*schema*.*operatorname*)
> >
> > So introducing VARIABLE(schema.variablename) syntax as an alternative
> > syntax for accessing variables I really like. I strongly prefer to use
> this
> > as only alternative (secondary) syntax, because I don't think it is
> > friendly syntax or writing friendly, but it is safe, and I can imagine
> > tools that can replace generic syntax to this special, or that detects
> > generic syntax and shows some warning. Then users can choose what they
> > prefer. Two syntaxes - generic and special can be good enough for all -
> and
> > this can be perfectly consistent with current Postgres.
>
> As far as I recall, last time this topic was discussed in hackers, two
> options were proposed: the one with VARIABLE(name), what you mention
> here; and another one with adding variables to the FROM clause. The
> VARIABLE(...) syntax didn't get much negative feedback, so I guess why
> not -- if you find it fitting, it would be interesting to see the
> implementation.
>
> I'm afraid it should not be just an alternative syntax, but the only one
> allowed, because otherwise I don't see how scenarious like "drop a
> column with the same name" could be avoided. As in the previous thread:
>
>     -- we've got a variable b at the same time
>     SELECT a, b FROM table1;
>

I am sorry, but I am in very strong opposition  against this idea. Nobody
did reply to my questions, that can change my opinion.

1. This introduces possible inconsistency between LET syntax and SELECT
syntax.
What will be the syntax of LET?

LET var = var FROM var

PLpgSQL does something, and it is really strange, and source of some
unwanted bugs. See https://commitfest.postgresql.org/50/5044/

With current design I can support

LET var = expr with wars

or

LET var = (QUERY with vars)

It is perfectly consistent. The expressions will be expressions.

2. I don't know of any product in the world that introduced the same
requirement. So this syntax will be proprietary (SQL/PSM it really doesn't
require) and shock for any users that know other databases. Proprietary
syntax in this area far from syntaxes of other products is hell. Try to
explain to users the working with OUT variables of Postgres's procedures
and functions. And there is some deeper logic.

3. There is a simple solution - convention. Use your own schema like vars,
and use session variables in this schema, When this schema will not be on
the search path, then there is not a collision.
Variables living in schema. Nobody without CREATE right can create it. So
it is safe. Or use prefix in __ for variables in the search path.

4. this requirement introduces syntax inconsistency between plpgsql
variables and session variables - which breaks one goal of the patch -
introducing some global variables for plpgsql (and for all PL).

5. Using more variables and FROM clauses decrease readability of FROM clause

SELECT v1, v2, a, b, c FROM t1, v1, v2, t2, ...

6. Usually composite variables don't want to unpack. When you should use
FROM clause, then composite variables will be unpacked. Then all fields can
be possibly in collision with all other column name

Example

CREATE TYPE t1 AS (id int, name varchar)
CREATE TABLE tab(id int, name varchar)
CREATE VARIABLE var1 AS t1;

SELECT id, name, foo(var1) FROM tab, var1;

Now I have a collision in columns id, name, and everywhere I need to use
aliases. Without necessity to use var in FROM clause, I can just write

SELECT id, name, foo(var) FROM tab

and there is not any risk of collision




> Then dropping the column b, but everything still works beause the
> variable b got silently picked up. But if it would be required to say
> VARIABLE(b), then all fine.
>

but same risk you have any time in plpgsql - all time. I don't remember any
bug report related to this issue.


>
> And to make sure we're on the same page, could you post couple of
> examples from curretly existing tests in the patch, how are they going
> to look like with this proposal?
>
> About adding variables to the FROM clause. Looks like this option was
> quite popular, and you've mentioned some technical challenges
> implementing that. If you'd like to go with another approach, it would
> be great to elaborate on that -- maybe even with a PoC, to make a
> convincing point here.
>

There is not any problem with implementation. I see the main problem with
usability, and I really don't want to implement some like LET var = var
FROM var; I am sorry
It fixes one issue, but it increases possible collisions - so the variables
will be unusable.

Regards

Pavel