Reads, writes and how much data

Next, estimate the mix. Is the app mostly reading, like a blog or a course catalogue, or mostly writing, like a sensor logger or a chat? How many records will exist after a year: thousands, millions, billions?

How big is each one? Rough numbers are enough. A shop with 5,000 products and 200 orders a day is small for any modern database, and a single PostgreSQL instance will handle it for years.

A fleet of devices sending a reading every second is a very different job. Read-heavy apps benefit from caches and indexes. Write-heavy, append-only streams suggest time-series or columnar tools.