Cannot infer common string type for string concat operation Dictionary(Int32, Utf8) || Dictionary(Int32, Utf8)

[alamb]

Describe the bug

When I try to concat two dictionary encoded columns it doesn't work

To Reproduce

Concat constants

> 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

> 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 #12063 with @dharanad

I believe the fix will be to extend string_concat_coercion (also changed in #12063) to also handle dictionaries.

We can likely follow the model of dictionary comparison coercion:

datafusion/datafusion/expr-common/src/type_coercion/binary.rs

Line 886 in 121f330

fn dictionary_coercion(

[alamb]

I think this is a good first issue as there is a clear and small reproducer and the place in the code is known

It should also include some new sqllogictests

Here are the instructions: https://github.com/apache/datafusion/tree/main/datafusion/sqllogictest

Ideally you should be able to extend one of the existing test files in https://github.com/apache/datafusion/tree/main/datafusion/sqllogictest/test_files

(you could uncomment the test in string_view.slt that was added in #12063)

[thinh2]

take