Description:

Several levels of brutality when rotating connections:
Session pooling - Most polite method. When client connects, a server connection will be assigned to it for the whole duration it stays connected. When client disconnects, the server connection will be put back into pool.
Transaction pooling - Server connection is assigned to client only during a transaction. When PgBouncer notices that transaction is over, the server will be put back into pool. This is a hack as it breaks application expectations of backend connection. You can use it only when application cooperates with such usage by not using features that can break. See the table below for breaking features.
Statement pooling - Most aggressive method. This is transaction pooling with a twist - multi-statement transactions are disallowed. This is meant to enforce "autocommit" mode on client, mostly targeted for PL/Proxy.