question about about denormalization

Normalization makes sure that the data is not redundant, which is a good thing. But this can be bad as well.

Consider this example. Say there is a database for bank accounts. And imagine that there is a relation (table) for Account, and another relation for Transactions.

We do not need to have an ACCOUNT_BALANCE column, because we can always derive the account balance from summing up the transactions. But...

If we get the account balance in this way, every time the account balance is retrieved, a huge operation is performed on the database. When the number of Account records and Transaction records gets bigger, the computation gets intensive.

That's a good reason to de-normalize the database and have an ACCOUNT_BALANCE column in Account table.

I hope you get the idea.