High Water Mark active in Oracle

The APPEND hint works within statements performing DML insert operations from another table, that is, using a subquery from within an INSERT SQL statement. This is appropriate for when you need to copy a large volume of rows between tables. By bypassing the Oracle database buffer cache blocks and appending the data directly to the segment above the high-water mark, you save significant overhead. This is a popular method for inserting rows into a table very quickly. When you specify one of these hints, Oracle will perform a direct-path insert. In a direct-path insert, the data is appended at the end of a table, rather than using free space that is found within current allocated blocks for that table.
The APPEND and APPEND_VALUES hints, when used, automatically convert a conventional insert operation into a direct-path insert operation. In addition, if you are using parallel operations during an insert, the default mode of operation is to use the direct-path mode. If you want to bypass performing direct-path operations, you can use the NOAPPEND hint. Keep in mind that if you are running with either of these hints, there is a risk of contention if you have multiple application processes inserting rows into the same table. If two append operations are inserting rows at the same time, performance will suffer: since the insert append operation appends the data above the high water mark for a segment, only one operation should be done at one time. However, if you have partitioned objects, you can still run several concurrent append operations, as long as each insert operates on separate partitions for a given table.