INVISIBLE indexes are a feature in Oracle Database 12c and later versions that allow an index to be maintained but not used by the optimizer unless explicitly hinted.
A . False. An INVISIBLE index still consumes storage space as it is maintained in the background.
B . False. There is no such restriction. You can create multiple INVISIBLE indexes on the same column list.
C . False. The optimizer can consider INVISIBLE indexes if they are hinted at in the query.
D . True. You can alter the visibility of an index using the ALTER INDEX command to make an INVISIBLE index VISIBLE.
E . True. Even though they are invisible to the optimizer by default, all DML operations such as INSERT, UPDATE, and DELETE continue to maintain the index as they would with a VISIBLE index.
Reference:
Oracle Documentation on Managing Indexes: https://docs.oracle.com/database/121/ADMIN/indexes.htm#ADMIN11741