presto-main/src/main/java/io/prestosql/type/UnknownType.java/UnknownType/UnknownType
class UnknownType:    private UnknownType()
    {
        // We never access the native container for UNKNOWN because its null check is always true.
        // The actual native container type does not matter here.
        // We choose boolean to represent UNKNOWN because it's the smallest primitive type.
        super(new TypeSignature(NAME), boolean.class);
    }