út 10. 8. 2021 v 19:48 odesílatel Bryn Llewellyn <bryn@xxxxxxxxxxxx> napsal:
pavel.stehule@xxxxxxxxx wrote:tgl@xxxxxxxxxxxxx wrote:pavel.stehule@xxxxxxxxx wrote:Some errors like this, but not this can be detected by plpgsql_check
https://github.com/okbob/plpgsql_checkprobably the heuristic for type check is not complete.
STRICTMULTIASSIGNMENT would detect most cases of this, except that the condition is checked too late. We'd need to count the fields *before* trying to assign values, not after.
In the meantime, it does seem like the docs could be more explicit about this, and perhaps give an example showing the (x).* solution.
Yes, a more detailed explanation of this behavior can be nice. There can be an example of value unnesting, but I think so for this case, there should be mainly an example of ANSI assign syntax.
var := (SELECT x FROM ..)
This syntax has advantages so is not amigonuous for this case, and explicit unnesting is not necessary (and it is not possible). Moreover, this is ANSI SQL syntax.Consider this example:create type type1 as (a1 int, a2 int);
create table tab1(k int primary key, b boolean not null, t type1 not null);
insert into tab1(k, b, t) values(1, true, (10, 20));
select b::text, t::text from tab1 where k = 1;
It seems to be perfectly plausible—and so it seems equally plausible that you'd want to do it using PL/pgSQL. Each of these two alternatives, inspired by the advice that I got in this thread, works and produces the expected output:
Yes, this works. This syntax is not ambiguous.
do $body$
declare
r record;
begin
select ((b, t)).*
into r
from tab1
where k = 1;
raise info 'Alt 1: % | %', r.f1::text, r.f2::text;
r := (
select (b, t)
from tab1
where k = 1);
raise info 'Alt 2: % | %', r.f1::text, r.f2::text;
end;
$body$;
It feels a smidge uncomfortable because I'm forced to use reference by field position (f1, f2) rather than by field name (b, t). But reference by position is commonplace in PostgreSQL (for example, in the PREPARE statement). So I'spose that I have to learn to like it.
postgres=# do $$
declare r record;
begin
select 10 as a, 20 as b into r;
raise notice '% %', r.a, r.b;
end;
$$;
NOTICE: 10 20
DO
declare r record;
begin
select 10 as a, 20 as b into r;
raise notice '% %', r.a, r.b;
end;
$$;
NOTICE: 10 20
DO
The composite value always has structure, and types, but sometimes it can lose labels. You can push labels by casting
r := (select (b, t) -- this is dynamic composity value, but without labels - the scalar value doesn't hold label
or
r := (select (b, t)::type1 -- it is composite with labels again
Do you (all) think that the "Alt 1" and "Alt 2" that I show are the best way to implement the requirement? I prefer "Alt 2" because it doesn't have the clutter (and the burden for understanding and readability) of the extra parentheses and the ".*".
I prefer Alt 2 too.
I can get the reference by field name that I prefer with a schema-level type:create type tmp as (b text, t type1);and by declaring "r" with this data type. But this is a greater discomfort than using the dynamically shaped "record" because it needs you to create a dedicated schema-level type for every new SELCT list that you come need.
When It is possible I use a record type - some years ago, the work with this type was a little bit slower, but not now. The work with this type is little bit safer - because it gets real labels. Values with declared composite types uses positional assignment, that is not too safe.
create table foo(a int, b int);
create type footype as (a int, b int);
insert into foo values(10, 200);
postgres=# do $$
declare f footype;
r record;
begin
select b, a from foo into f;
select b, a from foo into r;
raise notice 'f.a: %, f.b: %', f.a, f.b;
raise notice 'r.a: %, r.b: %', r.a, r.b;
end;
$$;
NOTICE: f.a: 200, f.b: 10
NOTICE: r.a: 10, r.b: 200
DO
postgres=# select * from foo;
┌────┬─────┐
│ a │ b │
╞════╪═════╡
│ 10 │ 200 │
└────┴─────┘
(1 row)
declare f footype;
r record;
begin
select b, a from foo into f;
select b, a from foo into r;
raise notice 'f.a: %, f.b: %', f.a, f.b;
raise notice 'r.a: %, r.b: %', r.a, r.b;
end;
$$;
NOTICE: f.a: 200, f.b: 10
NOTICE: r.a: 10, r.b: 200
DO
postgres=# select * from foo;
┌────┬─────┐
│ a │ b │
╞════╪═════╡
│ 10 │ 200 │
└────┴─────┘
(1 row)
But sometimes explicit type is necessary - when you want to return composite value and when you want to work with composite outside function, or when you want to serialize, or deserialize composite value to/from json.
When you work with composite values, is good to enable warnings
Regards
Pavel
