That is actually worse, I've been there. It's good to keep logs like that, but you can't use that for locking, you need a separate balances table.

Edit: Well another option is to add a "pending" col and do three separate db xacts: 1. insert pending=true row 2. select balance with pending debits deducted (which ages out pending rows older than 1min) 3. update row to pending=false if successful. This is a useful pattern if you're waiting on an external system too, but not good in this case where you're just trying to update in one DB.

This is the answer for any serious banking/accounting software.

Balance is calculated & stored after the fact from a known correct value.