jonahgao commented on code in PR #15120:
URL: https://github.com/apache/datafusion/pull/15120#discussion_r1987556974
##########
datafusion/expr/src/expr.rs:
##########
@@ -392,11 +392,34 @@ impl Unnest {
}
/// Alias expression
-#[derive(Clone, PartialEq, Eq, PartialOrd, Hash, Debug)]
+#[derive(Clone, PartialEq, Eq, Debug)]
pub struct Alias {
pub expr: Box<Expr>,
pub relation: Option<TableReference>,
pub name: String,
+ pub metadata: Option<std::collections::HashMap<String, String>>,
Review Comment:
> yes, only columns/fields have metadata, and it will finally set field's
metadata. but how to use field's metadata directly?
>
> for type_coercion.rs, I can add more code to cover it.
I reconsidered it and realized that type_coercion doesn't need to be care
about it.
It is only used by `Expr::to_field`
https://github.com/apache/datafusion/blob/a176aacc92a5b0272690bb48e7084ff5603ef7ad/datafusion/expr/src/expr_schema.rs#L460-L467
Perhaps we should add some comments about its purpose and its connection
with `Expr::to_field`.
To provide the following convenient API, the current solution makes sense to
me.
```rust
df.select(vec![col("a").alias_with_metadata("b", Some(metadata))])?
```
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