Hidden Virtual Column
https://blogs.oracle.com/sql/ora-54033-and-the-hidden-virtual-column-mystery
select column_name, data_default, hidden_column
from user_tab_cols
where table_name = 'TAB';
COLUMN_NAME DATA_DEFAULT HID
----------- ------------ ---
SYS_STUYPW88OE302TFVBNC6$MMQXE SYS_OP_COMBINED_HASH("X","Y" YES
Z NO
Y NO
X NO
column_name [datatype] [GENERATED ALWAYS] AS (expression) [VIRTUAL]
https://oracle-base.com/articles/11g/virtual-columns-11gr1