Rebuild Index

An index rebuilt either Online or Offline.

Online Index Rebuild Features:
* ALTER INDEX REBUILD ONLINE;
* DMLs are allowed on the base table
* It is comparatively Slow
* Base table is referred for the new index
* Base table is locked in shared mode and DDLs are not possible
* Intermediate table stores the data changes in the base table, during the index rebuild to update the new index later
Sql> alter index index_name rebuild tablespace tablespace_name online

Offline Index Rebuild Features:
* ALTER INDEX REBUILD; (Default)
* Does not refer the base table and the base table is exclusively locked
* New index is created from the old index
* No DML and DDL possible on the base table
* Comparatively faster

Sql> alter index index_name rebuild tablespace tablespace_name

So, the base table is not referred for data when the index is rebuilt offline.

When an index is rebuilt offline there is no FTS on the base table. When index is rebuilt online all the blocks from the base table are accessed.

These conclusions are when we donot make an scenario when the index is unusable and then there
is data load to the base table, and finally the index is rebuilt.