alamb commented on issue #16903:
URL: https://github.com/apache/datafusion/issues/16903#issuecomment-3689591663
> Also encountered a similar case for substr, the return type of `substr` is
utf8view, so if we create a table with utf8view, then the column type will
silently turn into `utf8view`, though we expect it to keep the input column's
type -- utf8
>
> ```
> create table t as select substr(a, 5, 3) from t1; --t1.a is `uft8`
>
> show columns from t;
>
> datafusion public t substr(Utf8("a"),Int64(5),Int64(3))
Utf8View NO
> ```
Maybe we should just turn off that "optimization" for substr -- and maybe
formalize the invariant that functions should try to maintain the same type as
their inputs 🤔
For your particular case @xudong963 I think you could potentially workaround
the issue by override your `substr` method with one that outputs Utf8 when
given utf8 input
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