how can i get the sum of two colums realtime

i have the following

+----------+----------+
| a        | b        |
| numeric  | numeric  |
+----------+----------+
| 10       | 10       |
+----------+----------+
| 10       | 10       |
+----------+----------+

I want to have something like that

select *, a+b as mes from maths;

but to have a column sum and evrytime the columns a and b get update calculate the new sum in columns sum

i try this and i get the following error

Answer

The syntax for generated column in PostgreSQL is:

alter table maths add column kostas int generated always as (a + b) stored;

Demo: https://dbfiddle.uk/?rdbms=postgres_12&fiddle=7f328dbdb991e68b46a8292bf7c480ab