Recreating and Rebuilding Indexes

Rebuilding Index
Syntax: Alter index index_name rebuild.
If you rebuild the index, the existing index will be dropped and created immediately, and any overheads existing on the index wil be removed. The main advantage in rebuilding the index is, if the existing index is getting used in any of the queries, those queries still use the index, while the rebuilding is in process. As a result, whle rebuilding, if you use the query, query uses the index there by avoiding the full table scans.

Recreating Index
To recreate the index, you need to drop it first. As a result the queries that use the index no longer use the index unless and until the index is recreated.

ALTER INDEX REBUILD