Hello Christina Halim
Did you try the following command to find and delete stats data older than 30 days?
select count(snapshot_id ) from _SYS_STATISTICS.STATISTICS_ALERTS_BASE where snapshot_id < add_days(CURRENT_TIMESTAMP,-30) COUNT(SNAPSHOT_ID);
delete from _SYS_STATISTICS.STATISTICS_ALERTS_BASE where snapshot_id < add_days(CURRENT_TIMESTAMP,-30) COUNT(SNAPSHOT_ID);
Thanks,
Shaheer