Use case
CAST in ClickHouse work in different way than in other DBMS.

1. Nullable is the part of data type. When you cast Nullable(UInt8) to String, you will get String.
   In other databases nullability is kept with CAST.

2. You can specify "conversion instruction" instead of a type name inside CAST.
   Example: CAST(x AS UNSIGNED) will cast Int8 to UInt8, Int16 to UInt16 and so on.

Describe the solution you'd like

1. Implement cast_keep_nullable setting.
2. Add a notion of named conversion instructions.