Transaction vs Session Pooling Modes

Each PostgreSQL backend process costs memory (work_mem, catalog caches, plan caches) and CPU. A few thousand idle client connections can exhaust a server even when nothing is running.

PgBouncer sits between your app and PostgreSQL, multiplexing many client connections onto a small set of real server connections. The pool_mode setting decides when a server connection is handed back to the pool.

• session — server held for the client's whole session
• transaction — server held only for one transaction
• statement — server released after every statement

In session pooling, a server connection is assigned to a client when it connects and is only returned to the pool when the client disconnects.

This is the safest mode: the client gets a dedicated backend for its entire lifetime, so every PostgreSQL feature behaves exactly as if it connected directly. The cost is poor reuse — an idle but connected client still pins a server connection.