Backend & systems·Concurrency and coordination
you reuse a fixed set of open database connections instead of paying the handshake on every query.
Connection pool
A cache of established connections shared by application threads. It removes per-request connection setup and, more importantly, bounds how many connections you can throw at the database. Pool size is a capacity decision on the database's behalf: too many and you overwhelm it, too few and requests queue in the application where you may not be measuring.