Why PostgreSQL Idle Connections Remain After Vercel Deployments

If you are running a Next.js application on Vercel with PostgreSQL, you may have encountered this error:

remaining connection slots are reserved for roles with the SUPERUSER attribute

This issue often appears right after a redeploy or during traffic spikes, even when your application traffic is relatively low.

The Symptom

This behavior is expected in serverless environments.

Here is what happens behind the scenes:

Vercel spins up serverless function instances
Each instance creates its own PostgreSQL connection or connection pool
When the instance finishes, it may shut down without gracefully closing the DB connection
PostgreSQL keeps the connection alive until its timeout is reached

As a result, PostgreSQL still sees these connections as idle, even though the Vercel instance is gone.

Managed PostgreSQL services (such as Aiven) reserve a portion of connections for system and SUPERUSER roles.

When your application consumes all remaining slots, PostgreSQL rejects new connections with the error shown above.

This is not a PostgreSQL bug — it is a mismatch between serverless architecture and direct database connections.

PgBouncer acts as a lightweight connection pooler:

Vercel → PgBouncer → PostgreSQL

For serverless workloads, transaction pooling mode is recommended.

This issue often feels like a Vercel problem, but it is actually a well-known serverless pattern.

Direct PostgreSQL connections and serverless deployments do not scale well together.

If you are using PostgreSQL with Vercel, PgBouncer is not optional — it is a production requirement.

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