Updating 20 million rows

Hi,

I need to update a table as follows:

Update table_a

set Column_1 = 99 where column_1 is null/ some other value

However, the table has 20 million rows. What is the best way to do this in the fastest possible manner??

The database version is 10gR2. The table is range partitioned on the date_key, with 1 month's record in 1 partition.

Please let me know if you need any more information.

Thanks in advance!