UNION should unify types

Why it matters

This feature addresses the problem of type unification in UNION queries where columns have differing types that can't be unified by default. By introducing a Variant type as the common type, it ensures smoother query execution without type conflicts.

How to use it

To enable this behavior, set the setting use_variant_as_common_type to 1. For example:

SET use_variant_as_common_type = 1;
SELECT ... UNION ...

This forces the UNION operation to unify incompatible types using the Variant type.