PostgreSQL, MongoDB, and what "cannot scale" means

What about "cannot scale without downtime"? While all databases can scale vertically, increasing or decreasing CPU or memory resources requires a restart, which leads to downtime. All sessions must disconnect, the system restarts, and users reconnect and execute their first queries with a cold cache. This is far from ideal, especially when workload spikes—the very situation where scaling up is needed. Cloud-native databases that promote scalability typically scale horizontally without downtime. This involves adding or removing nodes and automatically resharding data without stopping the application. Such elasticity is key to the cost efficiency in the cloud: it lets you scale resources up or down with the workload, avoiding excess provisioning.