cant_change_runtime_param parameter “…” cannot be changed now — 55P02

Symptoms

A configuration parameter could not be changed at this moment because of the current context (for example, it can only change at the start of a transaction or session). PostgreSQL raises SQLSTATE 55P02 (cant_change_runtime_param).

• The parameter is settable, but not in the present state.
• Some parameters only change before any query in a transaction.
• Distinct from restart-only parameters.

What the server log shows

ERROR:  parameter "transaction_isolation" cannot be changed now

Why PostgreSQL raises this — what the manual says

As Section 19.1 Setting Parameters explains:

Some parameters take effect only in a specific context (for example only at the start of a session or before a transaction begins); attempting to change such a parameter at the wrong moment fails because it cannot be applied now.

Certain parameters are constrained by context (transaction-start only, etc.). When you try to change one outside the permitted window, PostgreSQL rejects it with 55P02.

Common causes

• Changing a transaction-scoped parameter after a query has run.
• Setting a parameter that is fixed for the current operation.
• Issuing SET at the wrong point in a transaction.

How to fix it

1. Issue the SET at the right time (e.g. immediately after BEGIN).
2. Use the dedicated syntax (e.g. BEGIN TRANSACTION ISOLATION LEVEL …).
3. Set it at session start or via postgresql.conf if it must be global.

Related & next steps

• SQLSTATE 55P02 — Cannot Change Runtime Parameter (full reference)
• Message: parameter cannot be changed without restarting the server
• Glossary: GUC parameter

Reference: PostgreSQL 18 Section 20.1 “Setting Parameters”.