alamb opened a new issue, #12101:
URL: https://github.com/apache/datafusion/issues/12101
### Describe the bug
When I try to concat two dictionary encoded columns it doesn't work
### To Reproduce
Concat constants
```sql
> select arrow_cast('foo', 'Dictionary(Int32, Utf8)') || arrow_cast('bar',
'Dictionary(Int32, Utf8)');
Error during planning: Cannot infer common string type for string concat
operation Dictionary(Int32, Utf8) || Dictionary(Int32, Utf8)
```
Concat columns
```sql
> create table t as values (arrow_cast('foo', 'Dictionary(Int32, Utf8)'),
arrow_cast('bar', 'Dictionary(Int32, Utf8)'));
0 row(s) fetched.
Elapsed 0.010 seconds.
> select column1 || column2 from t;
Error during planning: Cannot infer common string type for string concat
operation Dictionary(Int32, Utf8) || Dictionary(Int32, Utf8)
```
### Expected behavior
Both queries should result in the single string `foobar`
### Additional context
Found this working on https://github.com/apache/datafusion/pull/12063 with
@dharanad
I believe the fix will be to extend `string_concat_coercion` (also changed
in https://github.com/apache/datafusion/pull/12063) to also handle dictionaries.
We can likely follow the model of dictionary comparison coercion:
https://github.com/apache/datafusion/blob/121f330a6ccca008da4bf6ffc4efa4ffbf961fd7/datafusion/expr-common/src/type_coercion/binary.rs#L886
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