Re: Schema variables - new implementation for Postgres 15

pá 3. 3. 2023 v 21:19 odesílatel Dmitry Dolgov <9erthalion6@gmail.com>
napsal:

> > On Tue, Feb 28, 2023 at 06:12:50AM +0100, Pavel Stehule wrote:
> >
> > fresh rebase
>
> I'm continuing to review, this time going through shadowing stuff in
> transformColumnRef, IdentifyVariable etc. Well, that's a lot of leg work
> for rather little outcome :) I guess all attempts to simplify this part
> weren't successful?
>

Originally I wrote it in the strategy "reduce false alarms". But when I
think about it, it may not be good in this case. Usually the changes are
done first on some developer environment, and good practice is to disallow
same (possibly confusing) identifiers. So I am not against making this
warning more aggressive with some possibility of false alarms.  With
blocking reduction of alarms the differences in regress was zero. So I
reduced this part.



>
> Couple of questions to it. In IdentifyVariable in the branch handling
> two values the commentary says:
>
>     /*
>      * a.b can mean "schema"."variable" or "variable"."field",
>      * Check both variants, and returns InvalidOid with
>      * not_unique flag, when both interpretations are
>      * possible. Second node can be star. In this case, the
>      * only allowed possibility is "variable"."*".
>      */
>
> I read this as "variable"."*" is a valid combination, but the very next
> part of this condition says differently:
>



>
>     /*
>      * Session variables doesn't support unboxing by star
>      * syntax. But this syntax have to be calculated here,
>      * because can come from non session variables related
>      * expressions.
>      */
>     Assert(IsA(field2, A_Star));
>
> Is the first commentary not quite correct?
>

I think it is correct, but maybe I was not good at describing this issue.
The sentence "Second node can be a star. In this case, the
the only allowed possibility is "variable"."*"." should be in the next
comment.

In this part we process a list of identifiers, and we try to map these
identifiers to some semantics. The parser should ensure that
all fields of lists are strings or the last field is a star. In this case
the semantic "schema".* is nonsense, and the only possible semantic
is "variable".*. It is valid semantics, but unsupported now. Unboxing is
available by syntax (var).*

I changed the comment



>
> Another question about how shadowing warning should work between
> namespaces.
> Let's say I've got two namespaces, public and test, both have a session
> variable with the same name, but only one has a table with the same name:
>
>     -- in public
>     create table test_agg(a int);
>     create type for_test_agg as (a int);
>     create variable test_agg for_test_agg;
>
>     -- in test
>     create type for_test_agg as (a int);
>     create variable test_agg for_test_agg;
>
> Now if we will try to trigger the shadowing warning from public
> namespace, it would work differently:
>
>     -- in public
>     =# let test.test_agg.a = 10;
>     =# let test_agg.a = 20;
>     =# set session_variables_ambiguity_warning to on;
>
>         -- note the value returned from the table
>         =# select jsonb_agg(test_agg.a) from test_agg;
>         WARNING:  42702: session variable "test_agg.a" is shadowed
>         LINE 1: select jsonb_agg(test_agg.a) from test_agg;
>                                                          ^
>         DETAIL:  Session variables can be shadowed by columns, routine's
> variables and routine's arguments with the same name.
>         LOCATION:  transformColumnRef, parse_expr.c:940
>          jsonb_agg
>         -----------
>          [1]
>
>         -- no warning, note the session variable value
>         =# select jsonb_agg(test.test_agg.a) from test_agg;
>          jsonb_agg
>         -----------
>          [10]
>
> It happens because in the second scenario the logic inside
> transformColumnRef
> will not set up the node variable (there is no corresponding table in the
> "test" schema), and the following conditions covering session variables
> shadowing are depending on it. Is it supposed to be like this?
>

I am sorry, I don't understand what you want to describe. Session variables
are shadowed by relations, ever. It is design. In the first case, the
variable is shadowed and a warning is raised. In the second case,
"test"."test_agg"."a" is a fully unique qualified identifier, and then the
variable is used, and then it is not shadowed.

updated patches attached

Regards

Pavel