The cache is a copy

Shop pages call the product API, which reads the product — name, description, price — from the database on every page load. At lunchtime that is 2,000 reads a second, nearly all of them for the same few hundred popular products, and a product's price changes a few times a day. The database is at 90% CPU. Of those 2,000 reads a second, how many come back with something different from the same read a minute earlier?

• Almost none. Nothing on the page changes that often, so nearly every read repeats one the database has just answered.
• All of them. Every page load is a different shopper, so every read is new work.
• About half. A shop changes its prices about as often as people look at them.