too many connections for database “…”

SQLSTATE 53300 condition too_many_connections class 53 — Insufficient Resources severity FATAL

Reproduced & verified on PostgreSQL 14.23, 15.18, 16.14, 17.10 and 18.4 — identical message on every version.

Last reviewed 30 May 2025 · Reproduced live with the SQL on this page.

Symptoms

A database hit its per-database connection limit set by ALTER DATABASE … CONNECTION LIMIT. PostgreSQL refuses the new connection with SQLSTATE 53300 (too_many_connections).

The database’s active connections reached its CONNECTION LIMIT.
New connections to that database are rejected.
Other databases may still accept connections.

What the server log shows

FATAL:  too many connections for database "appdb"

Why PostgreSQL raises this — what the manual says

As the ALTER DATABASE reference (Parameters) explains:

The database has reached the maximum number of concurrent connections set by its CONNECTION LIMIT (ALTER DATABASE name CONNECTION LIMIT n); raise the limit or wait for open sessions to close.

Each database can carry a per-database cap on concurrent connections. When that database’s active connection count reaches the cap, PostgreSQL rejects further logins to it with 53300, regardless of the cluster-wide limit.

Common causes

The database’s CONNECTION LIMIT is set too low.
Connection leaks or too many concurrent clients.
Multiple apps sharing one database without pooling.

Relevant GUC parameters

Parameter	Default	Effect
`max_connections`	100	Cluster-wide cap; the per-database limit applies on top of it.

How to fix it

Raise the limit: ALTER DATABASE appdb CONNECTION LIMIT 300;.
Introduce a connection pooler to bound connections.
Fix leaks and right-size pool sizes per service.

Diagnostic query

SELECT datname, datconnlimit FROM pg_database WHERE datname = 'appdb';

Compare against current connections: SELECT count(*) FROM pg_stat_activity WHERE datname = 'appdb';

Related & next steps

Reference: PostgreSQL 18 — ALTER DATABASE.