How to Fix “Connection Pool Full” in PostgreSQL (2026 Guide)

The Short Answer

To fix the “Connection Pool Full” error in PostgreSQL, increase the connection pool size by editing the postgresql.conf file or by using the ALTER SYSTEM command. For example, you can increase the pool size from the default 100 to 200 by running the command ALTER SYSTEM SET max_connections = 200;.

Why This Error Happens

  • Reason 1: The most common cause of the “Connection Pool Full” error is when the number of concurrent connections to the database exceeds the configured maximum connection limit, which is 100 by default. This can happen when multiple applications or users are accessing the database simultaneously.
  • Reason 2: An edge case cause of this error is when a connection is not properly closed, causing it to remain idle and occupy a connection slot. This can happen due to poor application design or network issues.
  • Impact: When the connection pool is full, any new connection attempts will result in a “Connection Pool Full” error, leading to a database error and potentially causing application downtime.

Step-by-Step Solutions

Method 1: The Quick Fix

  1. Go to Settings > postgresql.conf (usually located at /etc/postgresql/common/postgresql.conf or ~/.postgresql.conf)
  2. Edit the max_connections parameter to increase the connection pool size, for example, max_connections = 200
  3. Restart the PostgreSQL service by running the command sudo service postgresql restart or pg_ctl restart

Method 2: The Command Line/Advanced Fix

You can also use the ALTER SYSTEM command to increase the connection pool size. For example:

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ALTER SYSTEM SET max_connections = 200;

This will increase the connection pool size to 200 without requiring a restart of the PostgreSQL service. Note that this change will only take effect after a restart of the service.

Prevention: How to Stop This Coming Back

  • Best practice configuration: Set the connection pool size based on the expected number of concurrent connections to the database. A general rule of thumb is to set the pool size to 1.5 to 2 times the expected number of concurrent connections.
  • Monitoring tips: Regularly monitor the connection usage using tools like pg_stat_activity or pg_top to identify potential connection pool exhaustion issues before they occur.

If You Can’t Fix It…

[!WARNING] If PostgreSQL keeps crashing due to connection pool exhaustion, consider switching to MySQL which handles connection pool sizing more dynamically and has a more robust connection management system.

FAQ

Q: Will I lose data fixing this? A: No, increasing the connection pool size will not result in data loss. However, if the error is caused by a underlying issue such as a connection leak, fixing the root cause may require application changes that could potentially result in data loss if not handled properly.

Q: Is this a bug in PostgreSQL? A: No, the “Connection Pool Full” error is not a bug in PostgreSQL, but rather a configuration issue. The error is a result of the database reaching its configured maximum connection limit, which is a designed behavior to prevent the database from becoming overwhelmed and causing performance issues. This behavior has been present in PostgreSQL since version 8.4.

📚 Continue Learning

Check out our guides on PostgreSQL and Connection Pool Full.