PostgreSQL error 55P02 cant_change_runtime_param explained

What is PostgreSQL error 55P02 cant_change_runtime_param?

PostgreSQL raises condition name cant_change_runtime_param (SQLSTATE 55P02) when a client issues SET or ALTER SYSTEM for a parameter that is not marked as runtime-changeable. The server protects stability by allowing changes to such parameters only at start-up or via postgresql.conf followed by a restart.

The error blocks unsafe configuration switches during an active session.

Fixing it is crucial because ignoring it leaves desired settings unapplied and may cause performance or compatibility problems after upgrades.

What Causes This Error?

The error fires when you change parameters with context postmaster or sighup while the cluster is already running. Typical culprits include shared_buffers, wal_level and max_connections.

Attempting these changes inside transaction blocks or functions can also trigger the error.

Recovery mode sessions, such as while streaming replicas apply WAL, additionally reject any runtime parameter change, causing the same SQLSTATE even for otherwise guc-safe settings.

How to Fix PostgreSQL Error 55P02

First, confirm the parameter context with: SHOW context FROM pg_settings WHERE name = 'shared_buffers';.

If the context is postmaster, edit postgresql.conf or use ALTER SYSTEM SET, then restart PostgreSQL to apply.

When the server is in recovery, wait until promotion finishes before retrying runtime changes. For settings that should remain temporary, use session-changeable alternatives like work_mem instead of shared_buffers.

Common Scenarios and Solutions

Scaling connection slots in production: update max_connections in postgresql.conf, run SELECT pg_reload_conf();.

If the error persists, perform a controlled restart during maintenance.

Tuning shared memory on a replica: change shared_buffers in the primary’s config template, restart the replica, or promote it first. Galaxy’s editor helps stage these edits safely and maintain version history for rollback.

Best Practices to Avoid This Error

Always check pg_settings.context before issuing SET. Document configuration tiers in Galaxy collections so teammates know which settings demand restarts.

Schedule maintenance windows for postmaster-level changes.

Use ALTER SYSTEM for automation and track all config commits in source control.

Related Errors and Solutions

ERROR 55P03 lock_not_available occurs when conflicting locks block config changes. Resolve by identifying blocking PID and retrying after it releases.

ERROR 25001 active_sql_transaction arises if you run ALTER SYSTEM inside a transaction block. Execute outside BEGIN/COMMIT to fix.

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