The difference between "dba_optimize" and "dba_sync":
Use "dba_optimize" to optimize a database, which usually consists of eliminating gaps between records created by deletes.
The "dba_sync" function will synchronize the view of the database in memory and its image on the disk. As you insert records, they may be cached in memory by the underlying engine. Other processes reading from the database will not see these new records until synchronization.