blaginin commented on code in PR #13452:
URL: https://github.com/apache/datafusion/pull/13452#discussion_r1847145537
##########
datafusion/expr-common/src/type_coercion/binary.rs:
##########
@@ -1138,27 +1138,44 @@ fn numeric_string_coercion(lhs_type: &DataType,
rhs_type: &DataType) -> Option<D
}
}
+fn coerce_list_children(lhs_field: &FieldRef, rhs_field: &FieldRef) ->
Option<FieldRef> {
+ Some(Arc::new(
+
Arc::unwrap_or_clone(Arc::clone(lhs_field)).with_data_type(comparison_coercion(
Review Comment:
I may have misunderstood your point, but I am pretty sure
`comparison_coercion` preserves dicts:
https://github.com/apache/datafusion/blob/22c1f54411a02009629b3a76a43bd4343add045d/datafusion/expr-common/src/type_coercion/binary.rs#L637
For example, this query correctly casts `Dict(Utf8)` to `Dict(LargeUtf8)`
```sql
select arrow_typeof(x) from (select make_array(arrow_cast('a',
'Dictionary(Int8, Utf8)')) x UNION ALL SELECT make_array(arrow_cast('b',
'Dictionary(Int8, LargeUtf8)'))) x;
```
Also, I think `type_union_resolution` is a bit more limiting than
`comparison_coercion`, and so, for example, if I switch to it, the following
two queries will stop working:
```sql
-- type_union_resolution can't cast nulls
select make_array(arrow_cast('a', 'Utf8')) x UNION ALL SELECT
make_array(NULL) x;
-- type_union_resolution can't handle large lists (or fixed lists)
select make_array(make_array(1)) x UNION ALL SELECT
make_array(arrow_cast(make_array(-1), 'LargeList(Int8)')) x;
```
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