SQLSTATE P0004Severity lowLab verified

Incident brief

PL/pgSQL assertion failure

A PL/pgSQL ASSERT whose condition evaluates to false raises this error with the assertion's message; it signals a violated invariant, not ordinary data validation.

Reproduced on PostgreSQL 18.4Verified 2026-07-24 (isolated lab, PostgreSQL 18.4)Reviewed by Verified against PostgreSQL 18.4 in an isolated lab environment

In 10 seconds

What: PL/pgSQL assertion failure
What triggers it: Run a PL/pgSQL block containing ASSERT <condition>, <message> where the condition is false at runtime.
The fix: Make the asserted invariant hold before the ASSERT (fix the data or the preceding logic).
Proof: Reproduced on PostgreSQL 18.4 → A DO block reproduces SQLSTATE P0004; a false ASSERT condition aborts with its custom message and names the ASSERT in the CONTEXT.

The fix

What to do right now

The immediate, application-level response to this error.

Make the asserted invariant hold before the ASSERT (fix the data or the preceding logic).
Reserve ASSERT for should-never-happen invariants; use RAISE or validation for expected bad input.

Fix — SQL

-- Ensure the asserted invariant holds.
DO $$ BEGIN ASSERT (SELECT count(*) FROM (VALUES (1)) v) = 1, 'ok'; END $$;

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 a PL/pgSQL block containing ASSERT <condition>, <message> where the condition is false at runtime.
2PL/pgSQL evaluates the ASSERT and finds the invariant violated.
3The block aborts with SQLSTATE P0004 and the assertion message, pointing at the ASSERT in the CONTEXT.

A routine asserted that a ledger had exactly one open row, and a state where that was untrue tripped the assertion.

Schema setup

-- The error is self-contained in one statement; no schema is required.
SELECT 'no schema needed' AS setup_note;

Failing a PL/pgSQL ASSERT

DO $$ BEGIN ASSERT (SELECT count(*) FROM (VALUES (1),(2)) v) = 1, 'ledger must have exactly one open row'; END $$;

The session continues normally

SELECT 'ok' AS session_after_error;

What PostgreSQL actually returned

Schema setup output

    setup_note    
------------------
 no schema needed
(1 row)

Failing a PL/pgSQL ASSERT output

ERROR:  ledger must have exactly one open row
CONTEXT:  PL/pgSQL function inline_code_block line 1 at ASSERT

The session continues normally output

 session_after_error 
---------------------
 ok
(1 row)

Related & next steps

Follow the thread

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

Verification

PG 18.4

Last verified: 2026-07-24 (isolated lab, PostgreSQL 18.4)
Reviewed by: Verified against PostgreSQL 18.4 in an isolated lab environment
Audit status: reviewed