Feature: TransformCastSchema need check load_can_auto_cast_to during construction

[sundy-li]

Summary

Description for this feature.

[youngsofun]

load_can_auto_cast_to is much more strict, only used for load, not suitable for here.

we may introduce another help function like https://github.com/databendlabs/databend/pull/16072 this closed pr was intended to be used in copy only too (replaced by load_can_auto_cast_to later)

but @andylokandy said

理论上 任何类型都可以互转
所以之前没有 can_cast_to ，cast 不过去是值的问题不是类型问题

I think since currently our cast (run_cast) do report error on some (src, dst) pairs. may be it is ok to add a can_cast_to which is consistent with run_cast to report error earlier when building pipeline, and check it when constructing TransformCastSchema

[sundy-li]

But run_cast will check the cast types and throw

  _ => Err(ErrorCode::BadArguments(format!(
                "unable to cast type `{src_type}` to type `{dest_type}`"
            ))
            .set_span(span)),

[andylokandy]

We can say that a value is not able to be cast to another type, e.g. CAST('a' AS INT), but we can not say that all String can not be cast to Int. The former will throw at evaluation, and the latter will throw at type checking.

[sundy-li]

The former will throw at evaluation, and the latter will throw at type checking.

The latter will not throw errors at type checking, let's add it.

It will always return Expr::Cast

fn check_cast<Index: ColumnIndex> ... {

 Ok(Expr::Cast {
            span,
            is_try,
            expr: Box::new(expr),
            dest_type: wrapped_dest_type,
        })
}

[sundy-li]

A simple case, the error is thrown during sql pipeline execution.

🐳 :) create table a (t decimal(15,2));
processed in (0.018 sec)

🐳 :) insert into a select * from b;
error: APIError: ResponseError with 1006: fail to auto cast column t (Variant NULL) to column t (Decimal(15, 2) NULL)
unable to cast type `Variant` to type `Decimal(15, 2)`, during run expr: `CAST(t AS Decimal(15, 2) NULL)`

[andylokandy]

The former will throw at evaluation, and the latter will throw at type checking.

I mean, if we agree that all types can convert between each other, i.e. We can say that a value is not able to be cast to another type, e.g. CAST('a' AS INT), but we can not say that all String can not be cast to Int.， we should not throw at type checking.