alamb commented on a change in pull request #8460:
URL: https://github.com/apache/arrow/pull/8460#discussion_r504969480
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File path: rust/datafusion/src/physical_plan/expressions.rs
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@@ -1985,9 +1982,10 @@ mod tests {
#[test]
fn invalid_cast() -> Result<()> {
- let schema = Schema::new(vec![Field::new("a", DataType::Utf8, false)]);
- let result = cast(col("a"), &schema, DataType::Int32);
- result.expect_err("Invalid CAST from Utf8 to Int32");
+ // Ensure a useful error happens at plan time if invalid casts are used
Review comment:
> If/when we get to that point, we'll have to think about what behaviour
we want DataFusion to use.
I think DataFusion now makes the distinction between "casting" (aka if the
user specifically requests to cast from one type to another) which can be lossy
and "coercion" (aka when casts need to be added explicitly so that expressions
can be evaluated (e.g. plus).
Coercion is designed not be lossy, but casting can be.
The downside is that Datafusion has to have another set of rules of what
type coercions are allowed (e.g. I need to add
https://github.com/apache/arrow/pull/8463 to properly support DictionaryArray
in DataFusion).
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