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I’ve come across a PHP forum software which updates its threads views each hour.
So each time you view a thread, a row is inserted to a threadviews table with the thread id, then a script runs once an hour and updates the actual views count in the thread table.
My question is, what’s the logic behind this? Why not just update the thread table (i.e. views = views + 1)?
updates in general is way slower than inserts, you can think of update as delete and insert. updates might require locking to support ACID compliance of the DBMS, with inserts you dont have any locking.
Also, due to concurrency, you dont want to lock the row, and wait for the update to finish, think of this, what happen, while you are updating, you get a new visitor, you will lose that visitor. This is called the lost update. On the other hand, the cron job aggregates the visits and updates once an hour, since that row is read only, write lock wont affect your reads during the update.