Add computed value as new column while also removing duplicates - postgresql

Question

Let's say I have the following table playgrounds:
serialnumber length breadth country
1 15 10 Brazil
2 12 11 Chile
3 14 10 Brazil
4 14 10 Brazil
Now, I want to add a column area to the table, that is essentially length*breadth.
Obviously, I can do this update:
UPDATE playground set area = length*breadth where country = 'Brazil'.
Using the above statement, I will have to unnecessarily compute length * breadth twice for serial number 3 and 4. Is there a way to add group by and minimize the amount of calculations?
Something like:
UPDATE playground set area = length*breadth where country = 'Brazil'
group by length, breadth;

Answer 1

The first thing to note is that you should not add the area as a column. Data items that happen to be the result of simple arithmetic operations do not need their own column.
The second point is that you don't need to worry about doing a multiplication operation once each for rows 3 and 4. That's almost zero effort for the server
Third point is that if you are worried about rows 3 and 4, that means they are duplicated, and duplicated data should not be in the database. Consider deleting duplicates as described here: https://wiki.postgresql.org/wiki/Deleting_duplicates

Answer 2

To answer your question:
Is there a way, I could add group by and minimize the amount of calculations?
SELECT DISTINCT ON (1,2,3)
length, breadth, country, length * breadth AS area
FROM playgrounds
ORDER BY 1, 2, 3, serialnumber;
This takes the row with the smallest serialnumber from each set of duplicates. Detailed explanation:
Select first row in each GROUP BY group?
But consider the #e4c5's answer and Pavel's comment first. Don't store functionally dependent values that can be computed on the fly cheaply. Just drop duplicate rows and use a view:
To permanently delete dupes with greater serialnumber:
DELETE FROM playgrounds p
WHERE EXISTS (
SELECT 1
FROM playgrounds
WHERE length = p.length
breadth = p.breadth
country = p.country
AND serialnumber < p.serialnumber
);
Then:
CREATE VIEW playgrounds_plus AS
SELECT *, length * breadth AS area
FROM playgrounds;
Related:
Clean up SQL data before unique constraint