update x set y = null takes a long time

Question

At work, I have a large table (some 3 million rows, like 40-50 columns). I sometimes need to empty some of the columns and fill them with new data. What I did not expect is

Answer 1

Is column Y indexed? It could be that setting the column to null means Oracle has to delete from the index, rather than just update it. If that's the case, you could drop and rebuild it after updating the data.

EDIT:

Is it just column Y that exhibits the issue, or is it independent of the column being updated? Can you post the table definition, including constraints?