DBA

Oracle, stores data in segments. Segments has free and used space. The mark(a logical mark) between free and used space can be called as high water mark(HWM). When new data filled, the mark moves. On deleting records, oracle does not move back HWM. That is, you have unused data but, HWM sees as if it were filled data. When you shrink table such as alter table move command, this HWM refreshes itself.

When you delete from a table, a DML operation, oracle undos the changing. That is you can move back, with some additional works. But when you truncate a table, a DDL operation, oracle moves back the HWM. So this is faster than normal delete clause. With delete, you will fire existing triggers whereas truncate does not.

Truncate is especially is used instead of dropping and recreating tables. with truncate, no objects will become invalid and grants are remains the same

What Is Bottleneck ?

Bottleneck is the slowest operations of the set of operations. It is important to identify in the bottleneck in the system to know what needs to be turned to be better Performance and scalabilty.

Some Examples :-

The I/O system may become a bottleneck on large database system.

If a database used more memory than what’s available,paging and swapping will become a bottleneck.

In a RAC system the interconnect may be overloaded (cannot handle all data transfers