Why does it take a long time to drop a function based index in Oracle? I have a function based index that takes 25 minutes to create on a table of 94 Million rows.

When I drop the index, it takes 18 minutes.

Why does it take so long? I would have thought the drop would have been almost immediate? I have noticed that after ~10 minutes, the index is removed from user_indexes, but the script still executes for another ~8 minutes.

The table is partitioned, and this is an Oracle 10g RAC install. If any more information would help, I can add it as required.

Any light shed on this would be greatly appreciated.
A function-based index adds a virtual column to the table (This column is then indexed). Dropping the index removes the virtual column, which leads to a cleanup that takes time (same amount of work as the removal of a non-virtual column).