jayzhan211 commented on PR #14283:
URL: https://github.com/apache/datafusion/pull/14283#issuecomment-2614154584
The function `coerce_types` is used exclusively within function handling.
For case expressions, `coerce_types` is not utilized. Instead, the function
`get_coerce_type_for_case_expression` is used, which internally relies on
`comparison_coercion` as the actual coercion function
```rust
pub fn get_coerce_type_for_case_expression(
when_or_then_types: &[DataType],
case_or_else_type: Option<&DataType>,
) -> Option<DataType> {
let case_or_else_type = match case_or_else_type {
None => when_or_then_types[0].clone(),
Some(data_type) => data_type.clone(),
};
when_or_then_types
.iter()
.try_fold(case_or_else_type, |left_type, right_type| {
// TODO: now just use the `equal` coercion rule for case when.
If find the issue, and
// refactor again.
comparison_coercion(&left_type, right_type)
})
}
```
Therefore, I think Comet also need to applies such coercion before calling
`CaseExpr::try_new`
In datafusion, `Null` is coerced to `Int32(Null)` in `type_coercion`.
```
[2025-01-26T00:36:20Z DEBUG datafusion_optimizer::utils]
resolve_grouping_function:
Projection: CASE WHEN foo.a IS NULL THEN NULL ELSE foo.b END
TableScan: foo
[2025-01-26T00:36:20Z DEBUG datafusion_optimizer::utils] type_coercion:
Projection: CASE WHEN foo.a IS NULL THEN CAST(NULL AS Int32) ELSE foo.b
END
TableScan: foo
[2025-01-26T00:36:20Z DEBUG datafusion_optimizer::utils] count_wildcard_rule:
Projection: CASE WHEN foo.a IS NULL THEN CAST(NULL AS Int32) ELSE foo.b
END
TableScan: foo
```
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