PgBouncer is a widely used, lightweight connection pooler for PostgreSQL.
Due to Postgres connection limits, it is often useful to use a connection pooler to limit server connections. When too many server connections are used, a server's performance and reliability can be impacted.
PgBouncer is already installed on Heroku PostgreSQL databases but connection pooling needs to be enabled.
Read about how to set up PgBouncer connection pooling on Heroku.
Additionally, Heroku has several best practices for configuring PgBouncer as well.