Postgres: How to set column default value as another column value while altering the table

Question

I have a postgres table with millions of record in it. Now I want to add new column to that table called "time_modified" with the value in another column "last_event_time". Running a migration script is taking long time , so need a simple solution to run in production.

Answer 1

Assuming that the columns are timestamps you can try:

alter table my_table add time_modified text;
alter table my_table alter time_modified type timestamp using last_event_time;