SQLSTATE 55P02Severity mediumLab verified

Incident brief

Cannot change runtime parameter

A session tried to SET a parameter that can only be changed by restarting the server. PostgreSQL rejected the SET instead of silently ignoring it.

Reproduced on PostgreSQL 16.14Verified 2026-07-16 (Docker lab, PostgreSQL 16.14)Reviewed by Verified against PostgreSQL 16.14 in an isolated lab environment

In 10 seconds

What: Cannot change runtime parameter
What triggers it: Run SET max_connections = 200; inside an ordinary session.
The fix: Use ALTER SYSTEM SET to write the new value, then actually restart the server — reloading the config file (pg_reload_conf()) is not enough for a postmaster-context parameter.
Proof: Reproduced on PostgreSQL 16.14 → SET max_connections = 200; inside a session was rejected with SQLSTATE 55P02, because max_connections is a postmaster-context parameter.

The fix

What to do right now

The immediate, application-level response to this error.

Use ALTER SYSTEM SET to write the new value, then actually restart the server — reloading the config file (pg_reload_conf()) is not enough for a postmaster-context parameter.
Don't assume pg_reload_conf() returning true means the change took effect — see the premium counterexample for proof it doesn't, for this parameter.

Fix — SQL

ALTER SYSTEM SET max_connections = 150;
-- then actually restart the server (not just pg_reload_conf())

Diagnose

See it live on the server

Run these against the affected instance to confirm the diagnosis before you act.

Standard triage — not specific to this error

These are canonical PostgreSQL system-catalog queries, shown as SQL to run. No sample output is attached because this is general triage, not a captured lab transcript.

This SQLSTATE does not have an error-specific live snapshot yet. These are the canonical system-catalog queries you run against the affected server to see the problem in real time — standard triage, not a reproduced transcript.

What is running right now

Active backends, how long each has been running, and what it is waiting on.

SELECT pid,
       state,
       wait_event_type,
       wait_event,
       now() - query_start AS running_for,
       left(query, 80)     AS query
FROM   pg_stat_activity
WHERE  state <> 'idle'
  AND  pid <> pg_backend_pid()
ORDER  BY running_for DESC NULLS LAST;

Who is blocking whom

Turn raw blocking PIDs into the actual queries on both sides of the wait.

SELECT blocked.pid    AS blocked_pid,
       blocked.query  AS blocked_query,
       blocking.pid   AS blocking_pid,
       blocking.query AS blocking_query
FROM   pg_stat_activity AS blocked
JOIN   LATERAL unnest(pg_blocking_pids(blocked.pid)) AS b(pid) ON true
JOIN   pg_stat_activity AS blocking ON blocking.pid = b.pid
WHERE  cardinality(pg_blocking_pids(blocked.pid)) > 0;

Locks that are still waiting

Every lock a backend has requested but not yet been granted.

SELECT l.pid,
       l.locktype,
       l.mode,
       l.granted,
       COALESCE(c.relname, l.transactionid::text) AS object
FROM   pg_locks l
LEFT   JOIN pg_class c ON c.oid = l.relation
WHERE  NOT l.granted
ORDER  BY l.pid;

Reproduce & verify

A real, single-session PostgreSQL reproduction

A literal transcript of SQL run against a live PostgreSQL instance in an isolated lab — the commands below are exactly what was executed.

1Run SET max_connections = 200; inside an ordinary session.
2SQLSTATE 55P02 is reported; the setting is unchanged.

One client: SET attempted against a postmaster-context parameter.

Schema setup

SELECT name, setting, pending_restart FROM pg_settings WHERE name = 'max_connections';

The rejected SET

SET max_connections = 200;

Confirming the session still works

SELECT 1 AS session_still_alive;

What PostgreSQL actually returned

Schema setup output

      name       | setting | pending_restart 
-----------------+---------+-----------------
 max_connections | 100     | f
(1 row)

The rejected SET output

ERROR:  parameter "max_connections" cannot be changed without restarting the server

Confirming the session still works output

 session_still_alive 
---------------------
                   1
(1 row)

Related & next steps

Follow the thread

Everything this error touches — jump straight to the sibling error, term, runbook, or parameter.

Related errors

53300 · Too many connections

Verification

PG 16.14

Last verified: 2026-07-16 (Docker lab, PostgreSQL 16.14)
Reviewed by: Verified against PostgreSQL 16.14 in an isolated lab environment
Audit status: reviewed