PostgreSQL Connection Limits and PgBouncer

Monitoring and Operations / Guide 42 of 47 / Intermediate

• OSUbuntu 26.04 LTS
• PostgreSQL18.6-0ubuntu0.26.04.1
• Host2 core / 4 GB, 400,000-row test table
• TimeAbout 20 min

reserved_connections is separate from superuser_reserved_connections and was added in PostgreSQL 16, for members of pg_use_reserved_connections. PgBouncer here is 1.25.1.

This guide includes

Use this when connections run out. This matters because raising max_connections is the wrong answer - it needs a restart, it costs memory per slot, and a pooler is what actually fixes it.

• counting the budget across four settings, every one needing a restart
• lowering it, and watching a reload fail to apply the change
• filling every slot, and reading the error PostgreSQL actually gives
• limiting a role rather than the whole server
• installing PgBouncer, configuring transaction pooling, and serving sixty clients from five backends

Before you start

• A cluster you can restart - max_connections needs one.
• The appdb database.

1. 1

   Count the budget

2. 2

   Lower it, and watch the reload not work

3. 3

   Fill every slot

4. 4

   Limit a role rather than the server

5. 5

   Install PgBouncer and point it at the cluster

6. 6

   Configure transaction pooling

7. 7

   Sixty clients, five backends

8. 8

   Put the cluster back

Official sources

• PostgreSQL 18 Documentation - The Statistics Collector
• PostgreSQL 18 Documentation - Viewing Locks
• PostgreSQL 18 Documentation - Explicit Locking
• PostgreSQL 18 Documentation - Error Reporting and Logging
• PostgreSQL 18 Documentation - Connections and Authentication
• PgBouncer Documentation