Create column with duplicate data in same table psql - sql

Question

Postgres database
I'm trying to find a faster way to create a new column in a table which is a copy of the tables primary key column, so if I have the following columns in a table named students:
student_id Integer Auto-Increment -- Primary key
name varchar
Then I would like to create a new column named old_student_id which has all the same values as student_id.
To do this I create the column and the execute the following update statement
update student set old_student_id=student_id
Which works, but on my biggest table it takes over an hour, and I feels like I should be able to use some kind of alternative approach to get that down to a few minutes, I just don't know what.
So what I want at the end of the day is something that looks like this:
+------------+-----+---------------+
| student_id | name| old_student_id|
+------------+-----+---------------+
| 1 | bob | 1 |
+------------+-----+---------------+
| 2 | tod | 2 |
+------------+-----+---------------+
| 3 | joe | 3 |
+------------+-----+---------------+
| 4 | tim | 4 |
+------------+-----+---------------+
To speed things up a bit before I do the update query, I drop all the FK's and Indices on the table, then reapply them when it finishes. Also I'm on an AWS RDS, so I have setup a param group which has synchronized_commits=false, turned off backups, and increased working mem a bit for the duration of this update.
For context this is actually happening to every table in the database, across three databases. The old ids are used as references for several external systems which reference these ids, so I need to keep track of them in order to update those systems as well. I have an 8 hour downtime window, and currently merging the databases takes ~3 hours, and a whole hour of that time is spent creating these ids.

Answer 1

If in the future you do not need to update old_student_id column then you can use virtual columns on PostgreSQL.
CREATE TABLE table2 (
id serial4 NOT NULL,
val1 int4 NULL,
val2 int4 NULL,
total int4 NULL GENERATED ALWAYS AS (id) STORED
);
During the inserting process, the total field will be set to the same value as the id field. But you can not update this field, because this is a virtual column.
Alternative method is a using triggers. In this case you can update your fields. See this example:
Firstly we need create trigger function which will be called before table inserting.
CREATE OR REPLACE FUNCTION table2_insert()
RETURNS trigger
LANGUAGE plpgsql
AS $function$
begin
new.total = new.val1 * new.val2;
return new;
END;
$function$
;
After then:
CREATE TABLE table2 (
id serial4 NOT NULL,
val1 int4 NULL,
val2 int4 NULL,
total int4 NULL
);
create trigger my_trigger before
insert
on
table2 for each row execute function table2_insert();
With both methods, you don't have to update many records every time.