How to diagnose and fix the 55P02 cant_change_runtime_param error code in Postgres.

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The 55P02 error code in PostgreSQL indicates a cant_change_runtime_param error. This error means that there has been an attempt to change a server configuration parameter that cannot be altered while the server is running. Certain PostgreSQL settings require a server restart to take effect, and trying to modify these settings at runtime will lead to this error.

To diagnose and fix this issue, you should:

  1. Identify the specific parameter you are trying to change.
  2. Check whether the parameter is one that can be changed at runtime or requires a server restart.

Here are some examples and sample code to explain and cover the possibilities:

Example 1: Attempting to Change a Non-Runtime Parameter with SET

If you try to change a parameter that requires a server restart using the SET command:

SET max_connections TO 200;

This will result in a 55P02 error because max_connections is a parameter that cannot be changed at runtime.

To fix this, you need to update the postgresql.conf file:

max_connections = 200

And then restart the PostgreSQL server:

sudo service postgresql restart

Example 2: Using ALTER SYSTEM for Non-Runtime Parameters

Another approach to change a non-runtime parameter is using the ALTER SYSTEM command:

ALTER SYSTEM SET max_connections TO 200;

This writes the change to the postgresql.auto.conf file, which is read at server start. You will still need to restart the server for the change to take effect:

sudo service postgresql restart

Example 3: Attempting to Change a Parameter in a Transaction Block

Some parameters cannot be changed inside a transaction block. If you try to do so, you’ll encounter the 55P02 error:

BEGIN;
SET TRANSACTION READ WRITE;
SET wal_level TO minimal;
COMMIT;

wal_level cannot be changed inside a transaction block. To fix this, you should issue the SET command outside of any transaction block:

SET wal_level TO minimal;

However, note that wal_level is also a parameter that requires a server restart, so the correct way to change it would be through the configuration file and a server restart.

General Tips

  • Check the PostgreSQL documentation to see if a parameter can be changed at runtime or requires a server restart.
  • Use the pg_settings view to check the context column, which indicates when a parameter can be changed (postmaster for those requiring a restart, sighup for those that can be reloaded, and backend for those that can be changed at runtime).
  • For parameters that require a server restart, make changes in the postgresql.conf file or use the ALTER SYSTEM command and then restart the PostgreSQL service.
  • Plan for server restarts during off-peak times to minimize disruption to users.

By understanding which parameters can be changed at runtime and which require a server restart, you can avoid the 55P02 cant_change_runtime_param error in PostgreSQL. When changing configuration settings, always refer to the official documentation and follow best practices for server maintenance and configuration updates.

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