How to fix Cannot change runtime parameter in PostgreSQL

Query pg_settings to understand when the parameter can be changed:

SELECT name, setting, context FROM pg_settings WHERE name = 'your_parameter';

The context column will show: postmaster, sighup, superuser, user, or other values that indicate when changes are allowed.

If context is "postmaster", edit postgresql.conf with the desired setting and restart PostgreSQL:

# Edit the configuration file
sudo nano /etc/postgresql/VERSION/main/postgresql.conf

# Restart PostgreSQL
sudo systemctl restart postgresql

Common postmaster parameters: max_connections, shared_buffers, port, superuser_reserved_connections.

If context is "sighup", edit postgresql.conf and reload without restarting:

# Edit the configuration file
sudo nano /etc/postgresql/VERSION/main/postgresql.conf

# Reload configuration (PostgreSQL stays running)
psql -U postgres -c "SELECT pg_reload_conf();"

Or use the system command:

sudo systemctl reload postgresql

If context allows "superuser" or "user" changes, verify you have the correct role:

-- Check current user
SELECT current_user;

-- For superuser-only parameters, connect as a superuser
psql -U postgres

-- Then set the parameter
SET parameter_name TO value;

If you need to grant SET privileges to other users, the superuser can do:

GRANT SET ON PARAMETER parameter_name TO username;

For PostgreSQL 9.4+, ALTER SYSTEM provides a safer way to change parameters persistently:

ALTER SYSTEM SET parameter_name = value;

-- Then reload configuration
SELECT pg_reload_conf();

ALTER SYSTEM writes to postgresql.auto.conf, which PostgreSQL reads in addition to postgresql.conf. For postmaster parameters, the server must still be restarted.