aweltsch commented on PR #14283:
URL: https://github.com/apache/datafusion/pull/14283#issuecomment-2613791296
I also looked into this a little bit. Here's my observations:
The `data_type` of a `CaseExpr` is the first non-null `data_type`. In the
`expr_or_expr` specialization this is only the same type as the else expression
if the _then_expr is null_. My impression is that @jayzhan211 's suggestion to
handle only the null value would thus make sense.
I personally would have applied the transformation
```rust
let e = &self.when_then_expr[0].1;
let then_expr = try_cast(Arc::clone(e), &batch.schema(),
return_type.clone())?;
let then_value = then_expr
.evaluate_selection(batch, &when_value)?
.into_array(batch.num_rows())?;
```
since to me this seems more obvious what's the intended goal, i.e. "make
sure that the then_expr returns the correct datatype for the case expression"
and seems to be a little bit more generic.
If the suggested fix here is to be accepted I would rather remove the
`expr_or_expr` specialization again, because from what I have seen the current
suggestion regresses on the `case_when: expr_or_expr` benchmark.
Also: I was surprised to see that I can't reproduce this issue when using
datafusion-cli, so from an end-to-end perspective the type coercion seems to
work somehow :thinking:
This is what I did:
```sql
> CREATE TABLE batch2(a int[]) AS VALUES ([1, 2, 3]), (null);
> SELECT CASE WHEN a IS NULL THEN NULL ELSE a END FROM batch2;
+--------------------------------------------------------+
| CASE WHEN batch2.a IS NULL THEN NULL ELSE batch2.a END |
+--------------------------------------------------------+
| [1, 2, 3] |
| NULL |
+--------------------------------------------------------+
```
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