Alter table (with large no. of data)

Hi, I have tried to alter table by adding one more column in it. However, it fails after some time by displaying the following message: Long Transaction Aborted... How can I solve this problem by not unloading and reloading the data? I've already drop all the indexes except one of this table. The one I haven't dropped is the system index generated by the unique key when creating the table, something like " 384_12". Thank you! Rgds, Samuel Tan